iPad Pro stuck on iOS 13.7
Solution 1:
The first generation iPad Pro definitely supports iPadOS 14.
Ensure that you have a working internet connection, and try Settings > General > Software Update again.
If that doesn't work, try shutting down the iPad and power it on again - and try Software Update again.
If that still doesn't work, check if you have a beta software profile installed on the iPad (if you have previously been testing beta versions of iPadOS).
If everything fails, you could restore the iPadOS software to the iPad from a computer.