how do you solve the error ‘list assignment index out of range’ in python?

Use this:

weeknumber = []
for i in range(28):
    weeknumber.append([])
    for j in range(35)):
        weeknumber[i].append(pd.Timestamp(date_range[i][j]).weekofyear)

For an explanation of why you get surprising results to your other approach, refer to this answer to your other question