So your main problem seems to be your inability to distinguish between singular and plural.
Singular = 'single quotes' = char
Plural = "double quotes" = string
You must pick one. You cannot, as you attempt to do on line 23, assign a single character to a string. You cannot compare a string to a character. You cannot mix and match between them.
And your pop function must still return the top element from the stack, not just remove it from the stack and throw it away.