I am having the same issue. 2019 amended return will not electronically file. When I try to convert/transmit, that line turns yellow in Homebase. I don't even get an error message.
Yes, all updates done. I did find a solution to mine at least. While the original filed return was open, I clicked on E-file to try transmitting yet again. I noticed the 8th item down the list says "Amended Return Ready to EF". It was NOT checked. I don't know why but when I checked it and tried transmitting again, it went through. Hope this might be helpful. Thanks!