I'm linking you to the TurboTax FAQ that explains how to file by mail. There is one specific point in the FAQ that explains why this "may" be happening. See if the instructions help you out.
"How do I print and mail my return in TurboTax Online?"
https://ttlc.intuit.com/replies/3401713
*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***
