After you file

Why must you wait until you have received your refund to file the amendment? TurboTax never asked for my 1095-A and I forgot to include it. My return was accepted by the IRS on 01/27 and when I check the "Where's My Refund? Tool" from the IRS website, it still says it is processing. Why can't I file an amendment before receiving the refund? I'm starting to think that I cannot receive my refund until I do so...