The merge target is not mapped in D365FO/ AX7.0

Error: The merge target $/DynamicsAX/Trunk/Main is not mapped.

Reason; Local path is missing for the branch.

Solution:

Map a local path for the target branch(Main).
-> Click on "Not mapped"

-> Select any local folder from the browse button. (In my case I created a new folder in C driver and mapped it )

->  Click Map -> Get the objects into your local folder.

That's it.