I can't check in an existing project to VisualStudio.com
I had exactly the same happen - mucked about a bit and got it working by adding a project of the same name to my VS Online page.
Try this:
- Open your web browser to your visualstudio.com homepage
- Under "Recent projects & teams"
- Hit "New"
- Use your "RadioButtonTest" as the project name and go through the creation flow
- Check in from VS