I think your timer is missing one or more requred parameters. The required parameters for a valid timer are:
timer <repetitions> <interval/delay> <command>
Example:
That's a valid timer: it executes once, after 3 seconds. On execution, it will echo the word "hi" to the active window.
And these will all produce the error message you get - because at least one parameter is missing:
/timer 1 3
/timer 1
/timer echo -a hi
/timer 1 echo -a hi
If you have problems in finding the error, show us your current code.