How do you put one note on top of a different note?

I have been trying to write stuff on MuseScore but I cannot put one note on top of a completely different note. For example: I put a quaver on the bottom of the bar and I try to put a semi quaver or a crotchet on top of the stave in the same position as the quaver but it never works. If anyone knows what I'm talking about, please help. Thank you.