Hi @jing,
The issue appears to be that you are calling npm truffle-flattener when you should use npx if you don’t want to install.
You can try npx truffle-flattener ./contracts/myToken.sol > ./contracts/myToken_flat.sol which will run truffle-flattener without installing it.
Otherwise as per @Skyge (
thanks as always) you can install globally and run.
Where possible I recommend doing multi-file verification using Truffle or Hardhat: Verify smart contract inheriting from OpenZeppelin Contracts