If you look at the output, it says
Traceback (most recent call last):
File "main.py", line 8, in <module>
while col_let <= num_cols:
TypeError: '<=' not supported between instances of 'str' and 'int'
So the problem is that you're trying to compare a string (col_let) to an integral number (num_cols), which doesn't make much sense.
I'm not sure what the program is supposed to do. Maybe that would help clearing out what's going on. Also, the variable names are not too great (what does col_let mean?)