I’ve delivered my release but made a mistake and need to add a track, how can I do it?

Adding tracks to a release once it's already been made final isn't possible. You would need to use the “Copy Release” function to create a new version of the release with a new catalogue number and UPC, you can then add the track to the new release and deliver that, and takedown the original incorrect version. 

You can find more information on deliveries and takedowns here.