You can't have an alias named timer
(anything). /timer is a mirc command that is allowed to have text appended to it to give the timer a name. You can also use, for example /timername
, to display the status of a timer named "name". That's where the "timer name is not active" message is from.
You need to add a name for your alias along the lines of...
mytimer timerName 0 30 msg $chan My message goes here.
($chan is valid in aliases called from events as well as from the commandline)