When you a supervisor or a regular employee, you might not be able to say yes on everything that they give, and that is okay. Sometimes you will have to say no and that is not a bad thing.
When I am teaching or when I am outside of teaching, I have to say no sometime because I might not know how to do it or I know that cannot put 100% in to the assignment.
Mangers and supervisors need to hold each other accountable but also hold themself accountable as well.