I looked at the system log on my iPad before I updated and I noticed that these lines might be helpful in figuring out what's going on. [DIAG_END] ...[scan]checkForAvailableSlowReleaseUpdatesWithPolicy | result=8406 error:SUCoreError(Scan):kSUCoreErrorScanNoUpdateFound(8406) | [kSUCoreErrorScanNoUpdateFound] no update found [DIAG_END] ...[scan]startCatalogDownload (su) | result=44 error:com.apple.MobileAssetError.Download(MobileAssetDownload):MADownloadNotFound(44) | Download failed as the server said the file was not found. (Catalog download for com.apple.MobileAsset.SplatSoftwareUpdate) [DIAG_END] ...[scan]checkForAvailableSlowReleaseUpdatesWithPolicy | result=8400 error:SUCoreError(Scan):kSUCoreErrorScanCatalogDownloadFailed(8400)_1_com.apple.MobileAssetError.Download(MobileAssetDownload):MADownloadNotFound(44) | Download failed as the server said the file was not found. (Catalog download for com.apple.MobileAsset.SplatSoftwareUpdate)

I had the same issue with my iPad 9th gen. I restored it to the previous version with my computer and restored my data from a backup I made before I installed the beta.