Description
The branch for this instance is behind the intended target branch. Since beginning the release process work has been carried out (or merged into) the target branch (where you have selected your work to go).
MDOQ will only let you merge your instance in, if your is is only ahead, for more of an explanation see deployment/release process.
Troubleshooting
To resolve this simply merge the target branch back into your instance, and begin the release process again.