1. Do you have PvP enabled in your server.properties 2. What plugins do you have? (I'm guessing one of them puts a permission on pvp that you will have to give to players)
People can PvP, but only two OP Players can hit each other. I have the following plugins: WorldEdit WorldGuard CombatLog Essentials LuckyPerms Factions Duels Multiverse-Core Bounty
So one of those plugins disables pvp. Take a look at the documentation for each of those and see if any have permissions relating to PvP. Some that jump out at me that might, are WorldGuard, Essentials, and Multiverse-Core.