Did you create the 2021 fiscal year in GL yet? Might be a false message based on logic that it can't find the open period.

Just thought of another thing to check with is in Set Up AP Options and how you have configured the Current AP Year and Period and allow Unlock All Future Periods settings. If the future periods setting is checked and the number of future periods from current is exceeded then posting would be disallowed. Verify your current period in GL as well.

Thanks to the knowledge base and patience :), I figured out the issue.  In the fiscal calendar in the GL set up, AP was locked for all months of 2021 for 2 of the 3 companies I manage.  Not sure how it wasn't the same for all of them, but once I unlocked AP there, we were able to post.  Thank you very much for your responses though!

Yeah, that's weird and one of the last things I would have thought would occur in the creation of a new year either  automatically or manually. For those reviewing this, check your set up in Set Up AP Options as well as Set Up Fiscal Calendar in GL where you are able to review the locks for AP, AR, CM, IM, MF as well as the fiscal year status and configuration.