It's unlikely the file was created without an extension, even if it shows that way. Check the Get Info... dialog, under "Name & Extension". If that shows "2021 Taxes.tax2021", then the file already has the correct extension. (Very likely, with "Hide extension" checked underneath it.)
Unless you turn on the "Show all filename extensions" preference (Finder > Preferences... > Advanced), macOS won't necessarily show the extension for some files, but the name shown in the Info dialog is the full filename.
You're better off just unchecking "Hide extension" in that dialog, if you really want to see the extension, rather than trying to rename the file. And if it really doesn't have an extension, you can edit the name right there in that field of the Info dialog, to be sure you're editing the full name+extension.