Your error is probably that you forgot to cast the input(...) to int:
start_year = input('Enter start year')
start_year = int(start_year)
you should do the same for end_year and output.
Note: It’s really hard to try to help you without the source code. I need to infer a lot of things to help you diagnosis this error.
solved what to fix to solve the TypeError: an integer is required (got type str) [duplicate]