Apple released the second beta of iOS 15 and iPadOS 15 to developers. Unfortunately, as noted by 9to5Mac, iPadOS 15 beta 2 is not available for some models of the 9.7-inch iPad Pro.
The iPadOS 15 beta 2 release notes on the Apple Developer website mention that today’s release cannot be installed on the Wi-Fi + Cellular version of the 9.7-inch iPad Pro. The reason for the move is unknown, but Apple likely found a deal stopper issue and chose to disallow this release for the affected iPad models.
General. Known Issues: iPadOS 15 beta 2 isn’t available for iPad Pro (9.7 inch) (WiFi + Cellular). (79629736)
This means that users that are running iPadOS 15 beta 1 on this model iPad, won’t be able to update to beta 2. At least for now. Hopefully, Apple will come up with a fix by the time the third beta of iPadOS rolls out.