Calculate amount of days paid in advance and apply percent discount

I have a table, as part of an assesment task i am doing, with dates, and a course fee.
Part of the assesment task is to write a formula, to work out how many days in advance the customer paid, and then apply the needed discount. I have tried several basica variations to the formula, and keep getting the same Err message.
Can someone please point me in the right direction to how i can calculate amount of days paid in advance and apply a % discount?

attached is the start of the assesment question.
Thanks in advance for anyone who is able to help me

You should create and enter formulas to calculate the No. of Days paid in Advance, the Discount and the Course Fee Paid.
Use a VLOOKUP function in your template to determine the discount rate to be used for the calculation of the Discount. Your template should include a separate discount table containing the following information about the discount received:
• If students pay the course fee less than 7 days prior to the course commencing then they receive no discount.
• If students pay the course fee 7 to 13 days prior to the course commencing then they receive a discount of 5%.
• If students pay the course fee 14 to 20 days prior to the course commencing then they receive a discount of 8%.
• If students pay the course fee 21 days or more prior to the course commencing then they receive a discount of 10%.