Mendix Studio Pro (8 and above) uses tortoise SVN version 1.9, usually “One or more files are conflicted , please use external tool to resolve conflicts” appears when you have conflicts in the project files. you can refer to this documentation (https://docs.mendix.com/howto/collaboration-requirements-management/troubleshoot-version-control-issues)
Sometimes you have to restart the machine in order for the options to take effect in your folder. It has helped me quite a few times.
So you install SVN and then restart the machine and check if you have the option to revert.
Hope this helps!