You are building a swift CLI app, which means the main target cannot have tests directly associated.

The workaround described here is to define a second target and scheme to only use for testing. The Xcode Test for Mac Step, however, breaks on this type of scheme because it attempts to run the "build" action, which causes this error:

Make sure that the Run action is checked for the test scheme, as the error message suggests. You could also try to run the main scheme instead of the test scheme, since for that the Run action is checked by default.