This article explains how to find the number of courses your learners are currently enrolled in.
Apply any necessary filters, or open the Filter panel and click Apply without selecting filters.
Click on the Actions button at the top right and select Export. A csv file containing the report should be immediately downloaded onto your computer.
The csv file will contain a column called "Enrolled Courses" which lists the number of courses the learner has been enrolled in:
Filtering on this column and selecting 0 will allow you to see the list of learners which have not yet been enrolled in a course: