I am wanting to get an eSignature on a 2021 amended 1040 in ProSeries but am getting the error "None of the return(s) for the selected client are eligible for eSignature."

I've tried the following:

• Purchasing more eSignatures. It shows I have zero, but I did purchase two more.  
• Ensuring the box is checked to e-file the amended return.

I just successfully sent out an eSignature request for a 2020 return, so not sure why I can't do so for this 2021 amended.