UltraCosmetics supports both WorldGuard v6 (1.12-) and v7 (1.13+), and provides three WorldGuard flags you can use to control where players can use cosmetics.

uc-affect-players, when set to DENY, prevents players in the region from being affected by other players' cosmetics (for example the Quake Gun gadget or the Pig morph.)

uc-cosmetics, when set to DENY, prevents players from equipping or using cosmetics while in the area. Cosmetics are also automatically unequipped when entering the region. Also, players in this area cannot be affected by cosmetics, just like with uc-affect-players.

uc-blocked-categories is a list of cosmetic categories that cannot be used or equipped while in the region. (It's like a more granular version of uc-cosmetics.) Valid categories are: PETS, GADGETS, EFFECTS, MOUNTS, MORPHS, HATS, SUITS, and EMOTES. Cosmetics of blocked categories will also be unequipped when entering the region, just like with uc-cosmetics.

If you don't want UC to link with WorldGuard for any reason, you can disable it in the config under WorldGuard-Integration.

This is usually caused by either WorldGuard not starting correctly, or by another plugin doing something bad with its dependencies. For example, UltimateStacker is known to break UC's WorldGuard integration by trying to load both before and after WorldGuard, which obviously makes no sense.

To fix this, you should be able to manually remove the problematic loadbefore from the offending plugin's plugin.yml.