USSEP must be placed high in your load order, immediately after all the official master files and Creation Club content. Your load order should look like this:
Ensure all official DLCs are checked in your mod manager. If a newly installed mod specifically demands a post-1.6 version of USSEP, you will need to look for a legacy or backported version of that specific mod designed for 1.5.97. Issue: Red exclamation marks or missing textures in-game Unofficial Skyrim Special Edition Patch 1.5 97 Download
Fixing broken quest triggers, faulty scripts, and inconsistent NPC behavior. USSEP must be placed high in your load