That article in turn references Incremental Upgrade Scenarios for Jamf Pro 10.0.0 or Later which provides guidance on upgrading from Jamf Pro 10.10.1 to 10.20.1 (the requirements for 10.20.1 should cover 10.21.0 as well )
I know in the incremental article, it specifically states that going from 10.15.1 to 10.19 doesn't require any incrementals, but when I tried it on my server I ended up getting an error during startup/db updates that halted my upgrade. It was where the upgrade script could not find a column for certificate expirations.
I ended up rolling it back to 10.15.1, upgraded to 10.16.0, then 10.16.1, and finally up to 10.18.0. I still had to rename a few tables manually, but my error finding 'pending_revoke_certificate' column was cleared out and the upgrade went fine. Not saying it could happen to your instance, but since my db has been upgraded a few times starting with 9.91, it's worth keeping an eye on on the log.
If you need help, I can Zoom and help you. I manage my own instance currently on 10.21.0 and managed a few hosted instances too.
Also... are you trying to go from 10.10.0 to 10.21.0? I don't see 10.10.20 as a version number. Sounds like your Java and MySQL may be out of date halting the upgrade.