The cause of the issue is most likely that your mod system is not the correct version for applying the mod. Part of the installation is to patch the files for the correct mod system with the files found in "Put in Strive Game Files". Then the "backup" folder is to be deleted and then the game can be started.
As a side note, combatdata.gd should be included with the patch files and not with the mod files, as that version of the mod system is not capable of modding that file. Note, changing this may alleviate this specific error message, but it will not fix the core problem behind the error message so something else will take its place.