I had a similar problem and it was because I first created an Unit Test with the same class name. When I created the Instrumented Unit Test I got the error.
To solve it, I went to Edit Configurations, on the left of the run icon. Then below Unit Test, it was the ‘conflicting’ class, which I deleted. Click on Apply/Ok. Then I right click on the class name, click on run and voilà, it works.