I think you can file without the Letter 6475. The purpose of the Letter 6475 is to notify you how much the IRS thinks they sent you in 2021for the third Stimulus check. In my case my wife and I both received Letter 6475 showing that we each received $1,400 for a single Stimulus check for $2,800. That matched my records so I did not have to notify the IRS that I either received no check or was shortchanged in what I received. If you did not receive the third check at all or it was less than the proper amount, you need to file a 2021 Federal Tax Return and claim the Recovery Rebate Credit. The IRS is then supposed to check your claim and add to your refund if they think you are entitled to it. Either way you don't need to attach Letter 6475. (At least that is the way I understand it).
