No matter what I write in the console I get this message. What am I doing wrong? I am trying to enable l2tp-server like this:
How do I enter commands??

Are you sure that your "set" command syntax is correct?
Try to write "set ?" and you will see that "set" command has more fields to fill than you have entered.

you are missing a "sub-menu"...
it should be " /interface l2tp-server server " so as to see the server sub-menu. Imagine like going to interface --> l2tp-server --> server.
[aslint1@aslint] /interface l2tp-server server> set enabled=yes

as i can see you are trying to enable it as
[aslint1@aslint] /interface l2tp-server> set enabled=yes

all you have to do is write:
/interface l2tp-server server
set enabled=yes

and you are ready

Thanks for your replies. I was writing this wrong I guess. Now all commands work like a charm and I feel I very comfortable in the terminal

Good to know you made it work