[Note: This blog post applies to the nightly builds for ReSharper 5 version 1616 and lower]
If you’re getting the “Cannot Launch Debugger” message when trying to debug Unit Tests with the ReSharper runner, you’ll be happy to know it’s fixed. However, if you can’t wait until the next build, you can workaround by taking the following steps:
1. Locate the following files located in %Program Files%\JetBrains\ReSharper\v5.0\Bin\
- JetBrains.ReSharper.TaskRunner.CLR4.exe.config
- JetBrains.ReSharper.TaskRunner.CLR4.MSIL.exe.config
- JetBrains.ReSharper.TaskRunner.CLR4.x64.exe.config
2. Find the entry in each file:
<startup>
<requiredRuntime version="v4.0.21006" />
</startup>
and change them to:
<startup>
<requiredRuntime version="v4.0" />
</startup>
Hadi Hariri's Blog 
So I have Build 5.0.1659, have verified that the 3 config files all have v4.0 and I am still getting this message. Do you know of any other fixes?
this is quite old. It’s over a year ago. Have you tried 5.1.3?
Not yet. I’m not licensed for 5.1.3 so before I went and tried I wanted to see if you might know of any other solutions.
Thanks though.
I am using 5.1.3, or course registered and get the same problem. I am using Visual Studio 2010. I get “Cannot Launch Debugger” press “Ok” and run the debugger again and it works.
I think it’s best if you contact support at jetbrains dot com with your issue since this might be something specific to your machine or not a clean upgrade. This issue was solved over a year ago.