Phrases 'I will tell her not to do something' and 'you are supposed to not do...'

[Hara]

I would like to know why people sometimes say "I will tell her not to do something" and sometimes "you are supposed to not do something." What is the difference between the two? why in the first one not comes before to and the second not comes after to.

[Aphilash]

you are supposed to not do something Does it look a wrong usage (not aure)? Instead we would probably say "You are not supposed to do anything"