You'll have to check where your project is saved: if the path is too long, try to place the project elsewhere or shorten some parts of the directory. For instance, your path might be something like this:
C:\Installed Software\Mendix\SomeSuperlongfoldername\anotherfolder\anotherfolder\someveeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeerylongfoldername\Projects\YourProject-Main
In which case it would help to put the project in one of the higher level folders so that all the long folder names are removed from the path.