[Icecast] Jack trouble- cannot lock etc/passwd (Icecast/IDJC)

[Mavoy]

I`m making my radio station with Icecast. On Windows desktop everything
worked perfectly but on my private Ubuntu-based laptop it`s not so easy...
I` m using a tutorial here: 
http://idjc.sourceforge.net/tutorials_icecast.html
When I try to run Jack server I get something like this: "Jack is running in
realtime mode, but you`re not allowed to use realtime scheduling. Your
system has an audio group but you`re not a member of it. Please add yourself
to the group by executing as a root: usermod -a -G audio (my username)"

I wrote this line in the terminal but all I have now is:
usermod: cannot lock /etc/passwd try again later.

What it is? I`m afraid it may be something from the previous experiments
with Icecast- I had uninstalled everything that could be unnecessary but
something could stay. I`m an absolute greenhorn in Linux and I feel really
unsure in the terminal. Guys, if you have any ideas what`s going on please
write me fast. The official premiere of the radio station is planned on
Friday and it had been already delayed once - I don`t want to do it again.
:(
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