A cordial greeting to the entire community, excuse me, I'm looking to solve a problem and I need your help please!!
I am new to this section and I am learning, I have already deployed several contracts and I have verified them, always with Remix IDE...
The problem I have is that I have displayed the contract of an NFT collection without Flattening and when I check now it gives me an error, I have tried to flatten it in all ways and it still gives me an error, if you can help me it would be of great help. 
// SPDX-License-Identifier: MIT
pragma solidity >=0.7.0 <0.9.0;
import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol";
import "@openzeppelin/contracts/access/Ownable.sol";
contract NFT is ERC721Enumerable, Ownable {
using Strings for uint256;
string baseURI;
string public baseExtension = ".json";
uint256 public cost = 0.001 ether;
uint256 public maxSupply = 10000;
uint256 public maxMintAmount = 100;
bool public paused = true;
bool public revealed = false;
string public notRevealedUri;
constructor(
string memory _name,
string memory _symbol,
string memory _initBaseURI,
string memory _initNotRevealedUri
) ERC721(_name, _symbol) {
setBaseURI(_initBaseURI);
setNotRevealedURI(_initNotRevealedUri);
}
// internal
function _baseURI() internal view virtual override returns (string memory) {
return baseURI;
}
// public
function mint(uint256 _mintAmount) public payable {
uint256 supply = totalSupply();
require(!paused);
require(_mintAmount > 0);
require(_mintAmount <= maxMintAmount);
require(supply + _mintAmount <= maxSupply);
if (msg.sender != owner()) {
require(msg.value >= cost * _mintAmount);
}
for (uint256 i = 1; i <= _mintAmount; i++) {
_safeMint(msg.sender, supply + i);
}
}
function walletOfOwner(address _owner)
public
view
returns (uint256 memory)
{
uint256 ownerTokenCount = balanceOf(_owner);
uint256 memory tokenIds = new uint256;
for (uint256 i; i < ownerTokenCount; i++) {
tokenIds[i] = tokenOfOwnerByIndex(_owner, i);
}
return tokenIds;
}
function tokenURI(uint256 tokenId)
public
view
virtual
override
returns (string memory)
{
require(
_exists(tokenId),
"ERC721Metadata: URI query for nonexistent token"
);
if(revealed == false) {
return notRevealedUri;
}
string memory currentBaseURI = _baseURI();
return bytes(currentBaseURI).length > 0
? string(abi.encodePacked(currentBaseURI, tokenId.toString(), baseExtension))
: "";
}
//only owner
function reveal() public onlyOwner {
revealed = true;
}
function setCost(uint256 _newCost) public onlyOwner {
cost = _newCost;
}
function setmaxMintAmount(uint256 _newmaxMintAmount) public onlyOwner {
maxMintAmount = _newmaxMintAmount;
}
function setNotRevealedURI(string memory _notRevealedURI) public onlyOwner {
notRevealedUri = _notRevealedURI;
}
function setBaseURI(string memory _newBaseURI) public onlyOwner {
baseURI = _newBaseURI;
}
function setBaseExtension(string memory _newBaseExtension) public onlyOwner {
baseExtension = _newBaseExtension;
}
function pause(bool _state) public onlyOwner {
paused = _state;
}
function withdraw() public payable onlyOwner {
// Esto pagará al propietario el 100% del saldo del contrato.
// No elimine esto, de lo contrario no podrá retirar los fondos.
// =============================================================================
(bool os, ) = payable(owner()).call{value: address(this).balance}("");
require(os);
// =============================================================================
}
}