The Learner Reference Number (LRN) for a learner is displayed in the ILR section of their learner summary. To access this section, open the learner summary from the learner list and select ILR from underneath their name in the side menu.
The LRN will then be displayed on the left-hand side of the page, underneath the learner's display picture.
For more information, take a look at the full help page for the Learner ILR Summary.
Keywords: Where is learner LRN, learner reference code, finding reference number