Your wife DID file a return to get the 2020 stimulus payments so you must amend her return to add you and switch to MFJ since she should have filed MFS.
If you submitted the non-filer form, it was processed by the IRS as an original filed return. Any subsequent filing cannot be filed electronically; it has to be mailed in. If you file your return with a Form 1040X it will be processed like an amended return.
