If KRNL-Injector failed to find the Roblox process, delete KRNL App data and see. If that doesn't help follow the solutions mentioned in this post to resolve the issue.

Uninstall Roblox: Go to Settings > Apps > Apps & Features, find Roblox, and select Uninstall. Restart: Restart your device. Reinstall: Open the Microsoft Store, search for Roblox, and reinstall it.