Failing to verify contract on Etherscan from Remix

Hi all, I deployed a contract and I'm having trouble verifying via Etherscan. I used Remix to compile and deploy this contract.

A gist of the code I'm trying to get verified is https://gist.github.com/jonathonyule/9794fe274aafb8027a4bf3ad064df387

The error(s) are as follows:

ParserError: Source "@openzeppelin/contracts/security/ReentrancyGuard.sol" not found: File import callback not supported
  --> contracts...MuniGolfClub.sol:36:1:
   |
36 | import "@openzeppelin/contracts/security/ReentrancyGuard.sol"
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

ParserError: Source "@openzeppelin/contracts/access/Ownable.sol" not found: File import callback not supported
  --> contracts...MuniGolfClub.sol:37:1:
   |
37 | import "@openzeppelin/contracts/access/Ownable.sol"
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

ParserError: Source "@openzeppelin/contracts/utils/Counters.sol" not found: File import callback not supported
  --> contracts...MuniGolfClub.sol:38:1:
   |
38 | import "@openzeppelin/contracts/utils/Counters.sol"
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

ParserError: Source "@openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol" not found: File import callback not supported
  --> contracts...MuniGolfClub.sol:39:1:
   |
39 | import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol"
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

ParserError: Source "@openzeppelin/contracts/token/ERC20/ERC20.sol" not found: File import callback not supported
  --> contracts...MuniGolfClub.sol:40:1:
   |
40 | import "@openzeppelin/contracts/token/ERC20/ERC20.sol"
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

ParserError: Source "@openzeppelin/contracts/token/ERC20/IERC20.sol" not found: File import callback not supported
  --> contracts...MuniGolfClub.sol:41:1:
   |
41 | import "@openzeppelin/contracts/token/ERC20/IERC20.sol"
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

ParserError: Source "Base64.sol" not found: File import callback not supported
  --> contracts...MuniGolfClub.sol:42:1:
   |
42 | import "./Base64.sol"
   | ^^^^^^^^^^^^^^^^^^^^^^

ParserError: Source "remix_tests.sol" not found: File import callback not supported
 --> tests...4_Ballot_test.sol:4:1:
  |
4 | import "remix_tests.sol" // this import is automatically injected by Remix.
  | ^^^^^^^^^^^^^^^^^^^^^^^^^

ParserError: Source "contracts/3_Ballot.sol" not found: File import callback not supported
 --> tests...4_Ballot_test.sol:5:1:
  |
5 | import "../contracts/3_Ballot.sol"
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I have tried truffle-flattener as well but i get errors like below that prevent progress from trying to upload a flattened file as well.

{"name":"solidity-flattener","hostname":"computer.local","pid":38721,"level":40,"msg":"!!! @openzeppelin/contracts/security/ReentrancyGuard.sol SOURCE FILE WAS NOT FOUND. I'M TRYING TO FIND IT RECURSIVELY !!!","time":"2021-11-28T07:06:26.150Z","v":0}
{"name":"solidity-flattener","hostname":"computer.local","pid":38721,"level":30,"msg":"@openzeppelin/contracts/security/ReentrancyGuard.sol SOURCE FILE WAS FOUND","time":"2021-11-28T07:06:26.422Z","v":0}
{"name":"solidity-flattener","hostname":"computer.local","pid":38721,"level":40,"msg":"!!! @openzeppelin/contracts/access/Ownable.sol SOURCE FILE WAS NOT FOUND. I'M TRYING TO FIND IT RECURSIVELY !!!","time":"2021-11-28T07:06:26.422Z","v":0}
{"name":"solidity-flattener","hostname":"computer.local","pid":38721,"level":30,"msg":"@openzeppelin/contracts/access/Ownable.sol SOURCE FILE WAS FOUND","time":"2021-11-28T07:06:26.635Z","v":0}
{"name":"solidity-flattener","hostname":"computer.local","pid":38721,"level":40,"msg":"!!! @openzeppelin/contracts/utils/Counters.sol SOURCE FILE WAS NOT FOUND. I'M TRYING TO FIND IT RECURSIVELY !!!","time":"2021-11-28T07:06:26.635Z","v":0}
{"name":"solidity-flattener","hostname":"computer.local","pid":38721,"level":30,"msg":"@openzeppelin/contracts/utils/Counters.sol SOURCE FILE WAS FOUND","time":"2021-11-28T07:06:26.818Z","v":0}
{"name":"solidity-flattener","hostname":"computer.local","pid":38721,"level":40,"msg":"!!! @openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol SOURCE FILE WAS NOT FOUND. I'M TRYING TO FIND IT RECURSIVELY !!!","time":"2021-11-28T07:06:26.818Z","v":0}
{"name":"solidity-flattener","hostname":"computer.local","pid":38721,"level":30,"msg":"@openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol SOURCE FILE WAS FOUND","time":"2021-11-28T07:06:27.028Z","v":0}
{"name":"solidity-flattener","hostname":"computer.local","pid":38721,"level":40,"msg":"!!! @openzeppelin/contracts/token/ERC20/ERC20.sol SOURCE FILE WAS NOT FOUND. I'M TRYING TO FIND IT RECURSIVELY !!!","time":"2021-11-28T07:06:27.028Z","v":0}
{"name":"solidity-flattener","hostname":"computer.local","pid":38721,"level":30,"msg":"@openzeppelin/contracts/token/ERC20/ERC20.sol SOURCE FILE WAS FOUND","time":"2021-11-28T07:06:27.230Z","v":0}
{"name":"solidity-flattener","hostname":"computer.local","pid":38721,"level":40,"msg":"!!! @openzeppelin/contracts/token/ERC20/IERC20.sol SOURCE FILE WAS NOT FOUND. I'M TRYING TO FIND IT RECURSIVELY !!!","time":"2021-11-28T07:06:27.230Z","v":0}
{"name":"solidity-flattener","hostname":"computer.local","pid":38721,"level":30,"msg":"@openzeppelin/contracts/token/ERC20/IERC20.sol SOURCE FILE WAS FOUND","time":"2021-11-28T07:06:27.413Z","v":0}
{"name":"solidity-flattener","hostname":"computer.local","pid":38721,"level":40,"msg":"!!! /Users/jonathonyule/municipal-contract-truffle/node_modules/@poanet/solidity-flattener/contracts/Base64.sol SOURCE FILE WAS NOT FOUND. I'M TRYING TO FIND IT RECURSIVELY !!!","time":"2021-11-28T07:06:27.413Z","v":0}
glob error [Error: EPERM: operation not permitted, scandir '/Users/jonathonyule/Library/Accounts'] {
  errno: -1,
  code: 'EPERM',
  syscall: 'scandir',
  path: '/Users/jonathonyule/Library/Accounts'
}
node:internal/process/promises:246
          triggerUncaughtException(err, true /* fromPromise */);
          ^

[Error: EPERM: operation not permitted, scandir '/Users/jonathonyule/Library/Accounts'] {
  errno: -1,
  code: 'EPERM',
  syscall: 'scandir',
  path: '/Users/jonathonyule/Library/Accounts'
}

Any help would be greatly appreciated.

Did you try Flattener in Remix?

This worked, thank you!

Please search before posting on this forum. This is answered at length in our megathread on verification: