I'm in the same boat - can't efile an amended 2019 tax return. If the IRS will accept an efiled amendment for 2019, why can't TurboTax do it? I'm about to try printing out the amendment and entering all of the info in another free efile site to see if they will do it. It really upsets me that TurboTax won't provide this service when I paid good money for several years now to file my taxes.
