I'm training beta coherence and I'm getting little or no coherence values

Try switching to a different coherence type.

On Feedback Control, if "Training/Similarity" is selected, select "Assessment/Pure" instead.

The "Pure" coherence is insensitive to phase, and beta waves are often out of phase.

The "Training" coherence is more like a synchrony measure, and needs the signals to be in phase, like alpha waves.

For beta training, use "Assessment/Pure" coherence.