How do I know if I need to fix it? client / main: 813 & client / main: 801 Where?

Well it is an error so yeah. You should probably fix it.

It states in the error it is trying to get the length of (What I presume to be a table) that doesn’t exist or is set to nil on line 813 of a main.lua file.

And how do I know? Where is the destination?

With notepadd++ you can search into a folder, Search->Find in Files, in “Find what” write “trackedEntities” and in directory select your resources folder. Then you just have to press “Find All” and all reccurence will be marked.