The /jailcreate and the other commands are "unknown commands", if I type /help jail it says

Below is a list of all Jail commands:
/handcuff: Allows the player to handcuff another player
/jail: Jail specified player for specified amount of time.
/togglejaildebug: Toggles whether the plugin is in debugging state or not.
/unhandcuff: Allows the player to unhandcuff someone
/unjail: Release a player from jail.
/unjailforce: Remove player from the database.

That was all, so I can't create a jail. In debug mode there is no error, even without debug mode.
Thanks for helping.

I guess you are using jail 3.0? I believe 3.0 is still in beta and the commands are getting a complete overhaul.
You can find the new commands here: https://github.com/graywolf336/Jail/wiki/Commands

So only the blue commands will work?

No. The difference is this:
Old Jail create command: /jailcreate + arguments
New Jail create command: /jail create + arguments
Red just means there is no documentation available for that command yet (aka no Wiki article).

Would love for someone to submit a pull request and add a /jail help along with a /jailhelp command.