While .rpt file says Suspending not allowed in this context if you tried to suspend in an unscheduled environment, there's no certain error message in-game error message.
Suspending not allowed in this context Error in expression <sleep 1> Error position: <sleep 1> Error Generic error in expression
If it showed in-game, it will helpful for anyone who is lazy to look into .rpt (like me!) and new scripters.