This link might be useful for you to check your configuration.
Lightbulb
is an Equipment type , so you’re essentially telling openHAB ‘this is a group Item’ when you use the . Equipment Lightbulb
tagGroup Items don’t have Properties.
For Properties to work it needs to be a single Item, most often combined with a Point. Hopefully it’s clear in the link above!
EDIT: Removed misleading info.