This error means that characters 5-12 of the UPN are not all numeric values.
To check the pupil UPN please go to the student profile (search the pupil name in the top right hand search bar), go to the extended tab and under the school information heading please check the UPN here.
If the pupil is new to your school you may need to confirm with the previous school if you believe this number to be correct. You can find more information on UPNs from the DfE guidance found here.