Vesting contract throwing subtraction overflow error

Hello , I used the vestingvault contract but at the time of using addTokenGrant() , it throws an error of ‘subtraction overflow’ . I believe that is to say that owner does not have any token at the time of transfer to contract .
require(token.transferFrom(owner(), address(this), _amount));

but I provided all 1 billion token to owner in ERC20 contract at time of deployment. My contract vestingvault.sol is at

Can somebody kindly debug the issue ?.

Maybe, I am not sure, so could you please show the failed transaction hash?