It looks like you're trying to pass a specific value from your row into the isin() method. If you try using
if df.loc[n] in First_row that should get you past the first step. In any case the issue is definitely that you're giving isin() a value and not a list. Rather than embedding these loops super deep could you turn it into separate sections? I expect you'll run into trouble with i being reused like this as well, so iterating through each row separately will work better and you can just save i to a unique index variable. As it is your final printout will use the final value of i and print out every value what whatever index df.loc[n] is present in the third_row.