Thanks for the reply Emux
Using Cruiser (Mapsforge)...I tried debugging my map style by systematically deleting each section and then reloading the modified map style...nothing worked
However after turning off my PC completely and rebooting...Cruiser (Mapsforge) could load my Map Style without the error
I had the same problem with the android app loading my map style...again the same error "Negative Rule Requires Key"
I turned off my phone completely and rebooted...Cruiser app (Mapsforge) loaded my map style without any issues
Cheers
John