A learning track is purpose-built to contain and combine individual pieces of content into a cohesive pathway for learners. To keep things simple for the learners and ensure there is a clear path forward, a learning track can not be nested within another learning track.
The only content options for a learning track are: Articles, Files, Videos, Assessments, and SCORM.
A better option is to use a workflow to assign a learning track to a learner upon completion of a different learning track. So instead of nesting tracks, they are being modulized for easier consumption.
Step One: Navigate to Admin > Workflows and select the audience type. The best option would be A user has completed content or A user has completed an assignment. Choose whichever best applies to your situation, then enter the content that the learner will complete to trigger the next track.
Step Two: Under the Content tab, select the next track(s) to be assigned to the learner. For informal assignments where completion isn't being tracked, use the Share option.
Step Three: Finish completing the steps to create the workflow then be sure to activate it once you are ready for the workflow to go live.