Update IpadOs 13.5.1 on Ipad Pro

iPadOS 13.5.1 is an older version that's no longer signed by Apple, so you cannot update to that specific version. You can update to the latest version, iPadOS 13.6, following these instructions: Update your iPhone, iPad, or iPod touch

Get help with over-the-air iOS updates          

Have you tried using iTunes on a computer or Finder if running Catalina to do the update?

 Update software on iOS devices in iTunes on Mac

See If you see an error when you update or restore your iPhone, iPad, or iPod https://support.apple.com/en-us/HT201210  and

iOS update and restore errors https://support.apple.com/en-us/HT204770

Also see If your iPhone, iPad or iPod touch won’t update https://support.apple.com/en-gb/HT201435