If you haven’t created the student loan deduction first, it won’t be available to select when you process the pay run.
You only need to create a deduction for each student loan plan type once.
You don't need to add a value as Payroll will do the calculation for you.
If you're using HMRC notices, you’ll receive an SL1 notification. Once applied, Sage Payroll will assign the deduction to their pay.
Student loans