That's because Your server version 1.16.5. axolotls & glow squids are 1.17.1 mobs, change your server version to 1.17.1 to add them but 1.16.5 users will not be able to join. (i kno then how are you joining your minecraft server in 1.17.1 it's coz minehut wont let 1.17 items be generated even tho you can see in ur creative inv)