Verification smart contract using openzeppelin showing error

Facing some errors while following exact steps of I am trying to verify my deployed code using @openzeppelin/contracts.

I tried 1. truffle-flattener, 2. manually adding all files using multi-part files.
but it gives the same error again and again.

it says:

What tool did you use to deploy your contract?

truffle to compile & migrate.

Check out the Truffle section in this guide: Verify smart contract inheriting from OpenZeppelin Contracts