New versions of WorldGuard (7.0 or higher) have a very different API which makes simultaneously supporting legacy and 1.13+ WorldGuard builds impossible. Since MMOCore is natively built for 1.13 and higher, you must use this addon (drop it in the /plugins folder) if you're running a legacy spigot version with WorldGuard.
Make sure you have WorldGuard 6.2.2 as well as WorldEdit 6.1.8.