Update 9:20 PM 6/16/08: As of this evening, the coupon feature started working again. I was even able to add an ending date. I received the following when posting a new coupon:
“Your coupon was successfully created and saved.
Your new coupon should appear on the details page for your listing in Google Maps within a few minutes.”

Now if Google would just start promoting them! 🙂

The coupon creation function in the Local business Center has been broken for a number of days. Repeated reports of the inability to create or modify a coupon have been flowing into the Google Maps for Business and the Google Maps Trouble Shooting Groups. About June 10th, Coupons were able to be entered but stopped appearing in Maps, on June 11th attempts at entry failed and generated a message that the category and label you had entered were incorrect. Google, as of today, has not responded to the many queries in the Groups.

Google Coupons have been somewhat problematic since their inception. Google has never properly promoted nor displayed them adequately and expiration bugs have dogged the coupon system. As a result market uptake of Google Coupons has been erratic at best.

Given the update to the Local Business Center late last week, it is possible that the error message noting the new need for a category and label precedes a much needed upgrade to the Coupon feature.