If you get a "Mono signal detected" error message during the measuring process, please verify the signal flow. This error message should not be dismissed even if you're confident that everything is correct. The software can detect even the slight channel bleed, therefore, it's drawing your attention to a detected problem.
The "Mono signal detected" error message can be present even if the panning is off by a few percent (essentially, one of your speaker channels feeding into the other). The cause for this can also be found in the advanced properties of your interface control software (like some sort of enhanced Stereo mode, for example) while others can be caused by physical switch adjustments on your interface/other hardware in the chain. The point is - these sorts of adjustments might be buried deeper into your settings, even if the stereo image seems correct audibly. Other things worth checking:
- (XLR) cables. Test for signal and/or replace them.
- Flat surfaces (screen, window, desk, etc.) can make the audio bounce, which may 'confuse' the microphone. Try moving the speakers away from difficult surfaces.
- Subwoofer. When measuring with a subwoofer, make sure that the input/output are not defected/broken.