Is infinite inventory possible in multiplayer creative mode?
Solution 1:
You could try using the /gamemode <name> 1 command to set your users to be in creative mode. You'll need to do it for each user. Or you could do it like so: /gamemode @a 1 if you want all players to be in Creative mode.
Solution 2:
Set the world gamemode settings in server.PROPERTIES to 1, that way anyone who spawns is in creative mode.