The main things that causes this for most people is errors in installation process. So, double-checking that the correct files and folders are installed in the correct places is a main one.
Your particular error reminds me a lot of the conflict with the 'improved trees' addon. Unfortunately the various sub-mods for LotR-TW mostly do not play nice together. So if, for example, the Campaigns Addon is installed then it should be the *only* sub-mod applied.
Otherwise, detailed installation instructions (and various support posts below) can be found on the forum here:
Twcenter.net(How-to-install-on-Steam-Mac-Windows-10)
CapnDan
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