Hi there @CNP379 @M Barnett @jkalinich @jsnyder - 

Can you confirm that you have the extension box marked for e-file? 

From help article, How to file extensions in ProSeries
Go to Part VI - Electronic Filing  Information and check the Extension box for Federal Form 1065.

If the checkbox is marked and you continue to get this error, reply below so we can gather more information on this topic.