Here's a list of common reasons for this error to occur:
Improperly installed mods within dlcpacks (Ex. Installing a vehicle modification into patchday3 but it doesn't load correctly). To fix you must either remove those mods or fix the vehicles/dlcs not loading correctly.
Missing vanilla dlcpacks in your mods folder (Usually it will tell you something like "One or more of the required DLCs are not present. Refer to the game console for more information"). To fix this simply find the missing dlcpack (in your regular GTAV files) and while in edit mode select "Copy to Mods folder", if you don't get this option you may already have it in your mods folder but for some reason GTAV isn't detecting it in game. If GTAV isn't detecting a dlcpack in your mods folder it's usually because you have a mod improperly installed there, read above.
Realism Dispatched Enhanced (RDE). RDE and EUP aren't completely compatible in the current version of RDE, however it does seem to work with CERTAIN people. If you have RDE installed you must have World of Variety (WoV) installed for EUP and RDE to work together. Also note that for some people it helps to install EUP before RDE. If none of these work your problem may be with another mod, if you don't have any mods besides EUP and RDE it might be better to uninstall one or the other.
Wrong version of EUP or EUP Menu. Type "EUPMenuVersionInfo" in your RagePluginHook console (without the quotations of course), the versions should be EUP Menu 2.2.02 and EUP - Law & Order 7.5. If you have a different version for any of these you need to upgrade.
If you continue to see this error message I recommend joining our discord so someone from the Support team (most likely myself) can help you get your game fixed. I also recommend having TeamViewer because it's a lot easier for myself to help you fix your game, however it's not required. If all else fails you should probably start with a clean mods folder (But make a copy of your current one on your desktop). Hope this helps, good luck everyone!
https://discord.gg/aTQfepT