If your tax return has never been filed because the efile was rejected, you do not prepare an amended return form 1040X. All you need to do is print and mail your original return form 1040 to the IRS. If you have already started an amended return, you need to cancel that operation and go back and make sure your original return is correct: https://ttlc.intuit.com/questions/2552802-how-do-i-cancel-or-undo-an-amended-return-in-turbotax-onli...
What was the reason for the efile rejection? If it was because the 2018 AGI you entered didn't match the IRS records, you should try efiling again with the number 0 entered in place of the AGI.
If you just can't make efile work, go to the File section and in step 1 choose the print and mail option. When you print the return, TurboTax will print a filing instruction sheet that you can use for what to include and where to mail the return.
