The UKPRN field in ILR is populated from the Training Organisation that is linked to the learner. Please ensure that the learner is linked to a learning provider in Edit users page -> Learning provider field shown below.
Also check if the UKPRN has been populated for this Training organisation in the Organisation tile. If its blank, then enter the UKPRN and save the Organisation’s details, this UKPRN will then pull through to ILR.