Did you read the documentation here: https://docs.mendix.com/refguide/version-control#3-5-commit-changes-to-team-server-repository ? It give you some pointers on how to solve. My gut feeling would be to just revert the merge.
If you have tortoise SVN, you could go to your project folder right click → Tortoise SVN –> Check for Modifications
This will give you the list of files or folders. Will be helpful to understand what changes has happened.
Otherwise, revert the code and redo the procedure.
So your merge is unwanted and unneccesary. You probably have not committed the result of this merge yet. Simplest way is to delete you branch locally on your laptop and then redownload it again. The unwanted merge wil be gone and so will the conflict.
Now just try again: open your main branch, Version Control → Merge changes here → Select your branch