Source Control merge button grayed out?

Make sure to check the "File View" in the upper-left hand corner for more files that may have conflicts.

The default place you're looking at for resolving merge issues is the "Project View", which contains all project files. But especially if you are merging from/into another branch, there may be other more hidden (non-code) files that have been changed.

So switch to the "File View" to check for conflicts there as well. (it's located in the upper-left hand corner of the merge resolving screen)

I've experienced a similar issue after creating a branch and then attempting to 'merge branch into' the master.

In the window with the greyed out merge button I found if you deselect the blue highlighted branch to merge into (in my case 'master') by clicking in the space below, then select it again the merge button becomes available.

I know this doesn't answer why it's happening but as least merging can be used inside xcode again.

If using Xcode 11...

I found that SCM menu items were dimmed out, both in the "Source Control" menu and from within the Source Control Navigator (Pane 2).

Solution:

Go to the Source Control Navigator (Pane 2)

Select a Branch (e.g. "master"), then:

Editor -> Branch -> Merge "someBranch" in to "master"