That's not the command that the chat says to run.
1. I typed command: "/avatar bending add @p waterbending"
2. Then I press "Alt", because when i do it in singleplayer world i see my skills and CAN use them, but on server its chat me what i haven't any bending, but in list of my bendings(second screenshot) it says i have water bending already...
3. Maybe server can't to remember player's bending
4. I can't use my water skills