Automated Tests in Microsoft Test Manager "build directory of the test run either does not exist or access permission is required"

We are trying to run automated tests using Microsoft Test Manager 2015.

We've almost reached our goal. This is what was done so far:

• We are able to queue the 'build definitions' and successfully run Coded UI tests on a remote server.
• We've created a Test Environment and it's in a 'ready' state
• Test plan has build definition in use
• Test cases have association to the Coded UI tests.

More info:

• We don't have a build and copy to drop off folder steps because we don't have Visual Studio installed on the agent server
• So far, we manually put the .dll that contains coded UI tests to the C:\build folder on the agent server
• Test project resides in a separate solution from the actual project we are testing. and we want to keep it this way.

When we try to run a separate coded UI test that is associated with a test case using Microsoft Test Manager 2015, we get an error:

The build directory of the test run either does not exist or access permission is required

• I don't know how to specify the build directory. And we are not really building anything. My idea is just to run the tests from C:\build folder. Side note: Create Build Drop option is missing from our TFS 2015 vNext build definitions
• I've included Copy and Publish Build Artifacts step to the definiton, but I think it's useless because we are not even building anything. All DLLs are already on the drive
• I've checked the permissions for the C:\build folder. the account we are suing has full access to it.

Any help will be greatly appreciated