What permissions are needed to allow a user to Merge a Branch?

Using PDMpro 2018SP5.

One of my useres is able to create a branch, and if he views the Associated Branches window, and can right click and merge from there.

But the merge button under Tools/Merge is greyed out when he selects the file containing the merge.

With my admin permission, it is not greyed out when I select the file containing the merge.

I can't figure out what permissions would turn the merge button off, but still alow merge from the Associated Branches window.