Are you getting any error messages? Any feedback whatsoever?
Is it a surface model? STL format is only for solid models.
You won't get much of an answer with no info given.
Sorry for the lack of details.
It's a solid model, and it didn't have any error messages or anything like that. I just try to save it as an stl and the option to save it as an stl doesn't even show up.
I could attach the file if you want.
Attaching the file might get you some help. Depending on what version of Creo the file is saved in and whether it's an educational version of Creo or not, maybe someone will be able to get you an answer.