Here is the crucial detail:
: Solves progression-breaking bugs in major questlines, including the Civil War, Dark Brotherhood, and Main Quest. It ensures NPCs trigger dialogue correctly and quest items register in your inventory. unofficial skyrim special edition patch 1.6.318
LOOT displays a warning that your USSEP version is incompatible with your game version, even though you have 1.6.318 and USSEP 4.2.6a. Here is the crucial detail: : Solves progression-breaking