Why /gamerule does't work?

Hi Aternos Cummunity, I have a question. I created a server and added me OP, I go on Minecraft, I enter the server and use a /gamerule command,

for example /gamerule doFallDamage false (1.8.9 server) and I try if it works. It doesen't work, why?

Please help me,

your,

Kiro24.

Ok, some help?

yeah, why not.... MyGreenServer.aternos.me