Was the original return e-filed in ProSeries? " To electronically file an amended return the original return must have been filed electronically and have an Accepted e-file status." I believe this means you can only e-file a 1040X if  you e-filed the original. I know it was that way for 2019. Maybe @Just-Lisa-Now-   has some insight into this. 

https://proconnect.intuit.com/community/amend-tax-return/help/how-do-i-amend-an-individual-return-us...