What I think your teacher means is that you should restrict the user input to a number (the rows count) and a symbol - and that the symbol should be checked: it should be in the list you gave.
Once you have the character, you start printing the triangle.
That means reading the symbol inside a loop: if it's in the list, exit the loop - if it isn't, tell him what it can be, and go around again.
==
between the string the user input and a list of strings doesn't do that.
Python has an operator which checks if an item is in a list:
Python Membership Operators - in
and not in
[
^] which would help you.