This means your USSEP is too new for your game. Double-check that you downloaded version 4.2.5 (or the stated 1.5.97-compatible file). Also, verify your SkyrimSE.exe version is indeed 1.5.97.

Without USSEP, even a lightly modded Skyrim SE will eventually encounter bugs. With USSEP, the game becomes the polished experience Bethesda should have delivered at launch.

Implementation notes: