Can't upgrade from 1809

Might be, but it is the message users currently get when trying to force update a blocked system.

I have personally experienced it.

Might find out first what the OP is trying to upgrade to, and whether there is a block on the system.

Got the iso's for 1809, 1903, 1909.
Put on flash drive with Rufus.
1809 failed with the " This PC can't be upgraded to Windows 10" even though I'm using the same build.
1903 gets to 84% and hangs up. 1909 gets to 90%, same thing.

Do you think a clean install will work? Verified authentication and have the key just in case. Also, all data saved.

Yes, clean install should work.

With clean install you should not have to perform troubleshooting.
The time instead is spent installing new drivers and applications and restoring files.