I'm sorry. I posted the wrong screenshot of my error.
-Updated post
Can you narrow it down at all?
How many npc's do you have on the map? Is this error constantly popping up? Is it only when trying to interact with a certain npc? Or only when map loads? It seems like you are trying to use an npc script from a different datapack than the server you are running. Did you add npc's from an outside datapack recently?
The error gives us a lot of information, just not enough. We know for sure there is an error with "doNpcSellItem", my guess is that you copied from an old datapack the npcs, and brought them into your server. If this is so, you might be able to fix it simply by replacing the npc/lib folder with the one that...