A specific issue plagues Resident Evil 4 due to its various versions (the original 2005 release, the Ultimate HD Edition, and the 2023 Remake). Sometimes, the configuration file that tells the game which Steam AppID to look for becomes corrupted or missing.
Close Steam. Open Task Manager (Ctrl+Shift+Esc). End any "Steam (32 bit)" or "Resident Evil 4" processes. Restart Steam.
The "Steam must be running to play this game" error in Resident Evil 4 (Remake or HD)
Before diving into registry edits, verify these low-hanging fruits:
Many users don't realize that Steam installs a background Windows service ( Steam Client Service ) that handles DRM verification. If this service has crashed or is disabled, the game will always throw error messages.