Minecraft version 1.16.5 disable monsters server?

I'm recently playing with a friend Minecraft with the current version 1.16.5 on a server (only the two of us) my question now is how can I deactivate all monsters (only monsters) NO animals etc? In the network it says that you should do it in the server file and enter: spawn-monsters = "false" but unfortunately it didn't work…

Enter in chat or in the server console:

/ difficulty peaceful

If the entry in the chat does not work, you do not have the necessary OP rights. This can be given to you by typing in the server console:

op yourname

Set difficulty in the server.properties to 0.