I had a 2020 return that did that a few months ago, and I disable error checking (even though there were no errors) and it worked.  It might be worth a try.

There is another post on this forum and something else had worked for someone else, but unfortunately the search feature on this forum is useless.

If you have an attachment thats too big this will happen.

Make sure your program is updated (you may want to run Refresh/Repair updates) and try transmitting from the EFCenter, rather than from within the client file

How do I disable error checking?

I think you need to be transmitting from the EFCenter to turn off error checking, its right on that window that opens when you click the EFNow button...I dont think they give that option when you Efile from the client file.

Uncheck that box

Got it.  Thank you!

I unchecked the box; still unable to transmit ☹️.

NOTE: This is a 2017 Form 1040.  Does Pro-Series support e-filing for 2017?

Oh, its too late to Efile anything 2017.  The Efile window is only open for current year and 2 years prior at any time.  This is an IRS rule, not just Intuit.

Thanks! The attachment wasn’t an issue. Updated the program and I enabled error checking and it transmitted. 
now we wait to see if accepted. 

I think the real problem was the program update. 

I had the same thing this morning on a 2019 return.  No errors, went thru all the little steps, and ended up, right where i started, did the review again.... tried 5 or 6 times. then did an update on that year's program, and worked like a charm.