Numbers error in function with multiple operators

I'm having a hard time resolving an issue with what I thought was a basic calculation - but clearly I'm not doing something correctly.

I have a basic budget spreadsheet that i set up in a previous version of numbers.

In the left column I have a list of bills then a series of columns for the frequency of the payment, weekly, monthly, annually etc. I then have a column at the end that calculate the amount per fortnight. The original formula from the old version of Numbers is quite simple and works if I past it to another cell, however if I attempt to type the same formula I get an error - red triangle with an exclamation mark. I am certain I am overlooking something very basic, however I cannot figure this one out.

This is the formula that works. If I try and replicate the formula I get an error.

I can copy and paste the formula, update the cell reference and it works well. I can even change the fixed value numbers. Why would I be unable to type this in?

I have also tried with and without the SUM function.

Thanks in advance for helping me.