In one of your dataframes the year is a string and the other it is an int64
you can convert it first and then join (e.g. df['year']=df['year'].astype(int) or as RafaelC suggested df.year.astype(int))
Edit: Also note the comment by Anderson Zhu: Just in case you have None or missing values in one of your dataframes, you need to use Int64 instead of int. See the reference here.