According to some of the stuff people reported in the german forums the "problem" is that the stores want to get rid of their current items in stock first before getting the new promo-cans in. I checked the german thread regarding that promo just now and a few people reported that their respective stores got new cans (and promo-cans, even) in stock just yesterday. So maybe you have to try next week at some point today because you missed their re-stocking?

(Also, afaik the difference between REWE and REWE City is that the City-Store has less items in stock, focusseses more on the needs of someone who just goes to grab a few things in their lunchbreak and is more expensive.)