Hello Kenneth,
That DRO is set by default to display TrueRPM, which is only provided to Mach4 if you have an input set up to do so. You can change the DRO using the Screen Editor. Go to Operator>Edit Screen and click that DRO. You'll see the Properties section on the left-hand side. The DRO Code is set to Spindle TrueRPM, but you can change it there. I would recommend going to Screen>Save As and give your newly customized screen with a unique name. That will leave a clean default screen to revert back to and it will prevent your screen from being overwritten if you update later on.
Best wishes,
Bryanna