The letters slup sound like you are saying slump. Do not slump means: Sit up with your back strait. Slup is having a curved back in a chair. Sliding your hips forward to the end of the chair (with your head going further and further down the back of the chair.) A slump is also leaning forward way to far. YOU MAY SAY do not slip. and do not slump. With different meanings, of course.

Don't slip means to avoid making a mistake or falling slup is not a word. I am guessing you meant to write "SLURP" Don't "slurp" means do not eat or drink noisily.

Don't slup isn't a phrase... slup isn't even a word.

"Don't slip" is correct if you mean "avoid falling".