Cause: Your SKSE64 version does not match your USSEP. Solutions:
If you are playing on consoles:
File details to look for:
Unofficial Skyrim Special Edition Patch-266-4-2-5b-... This file was released in late 2021, right before the AE mass-update. Unofficial Skyrim Special Edition Patch 1.5 97 Download
If you are using a tool like "Skyrim SE Downgrader" to play on version 1.5.97 (common for specific mod lists like "Living Skyrim" or "Elderling" older versions), you cannot use the modern patch. Cause: Your SKSE64 version does not match your USSEP