Error 1550: UPN invalid (character 13 not a recognised value or is missing).

This error means that character 13 of the UPN is not a valid value. 

To check the pupil UPN:

1. Go to the student profile (search the pupil name in the top right hand search bar).
2. Go to the extended tab.
3. Under the school information heading 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.