The Position Tracking tool allows for you to receive ranking information for keywords SEMrush does not carry in their database. There may come times within your capaign that you will see a "n/a" symbol under Search Volume or CPC.
This means that the given keyword has too low of a search volume or CPC to be displayed. If any keyword has a search volume lower than 10, you will see an n/a symbol under that column.
However, if the keyword does have a higher search volume than 10 within SEMrush and you still see “n/a”, this means that a bad update was conducted on your project. This can happen from time to time in the software in which SEMrush will go to update your project and the rankings are not properly applied. This will fix itself within the next update which is conducted every 24-48 hours.
In order to learn more on creating a Position Tracking project, read this blog post.