A federal amended tax return can only be printed, signed, dated and mailed to the IRS. Federal amended tax returns can Never be e-filed. If you attempted to e-file the tax return that you changed from the original it will be rejected by the IRS since the original tax return has already been received.
Remove all changes you made to the original tax return.
Wait for the tax refund to be received from the original tax return or the taxes owed and paid to be processed by the IRS before starting the amend procedure.
See this TurboTax support FAQ for amending a 2018 tax return originally completed and filed using TurboTax -
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/01/27579
