I filed my original return with no problems, but got another form in the mail I needed to add to my tax return so I'm filling an amended return. I've already received my refund from the original return.
After I have added the new form to the amended tax return, I get this error message after trying to submit.
Since your Amended Return does not 'catch up' with your original return and correct it, you have up to three years to Amend.
Amended returns are processed separately, and can take several weeks to process.
I would advise trying to Efile your Amended return at a later date, or printing and mailing it.
You could try disabling anti-virus programs to see if it clears your transmission error. Here's How to Turn Off Anti-Virus Programs.