I don't follow you. I have libplain.so.2.0.22 in my system
and on the directory listing above as well ... (that's a
version number BTW)

Anyway, the command PB gave was this:

You have to be root to issue this command, so
that is why you start with the command

(after which, as before, whoami will confirm that you
are now root)

Frankly, it looks like you are not entering the command
exactly as it is specified (no "user" in it at all e.g.)

(What you are doing with this is creating what is called
a symbolic link from the already existing sasl libraries
to the place kmail is looking fro this libraries (which is
/usr/lib))