Sounds like you just did a zero benefit "shop". I would put it more crudely as would be appropriate, but you know what I mean. It happens.
Unfortunately there are getting to be a lot of those around these days. I did two last month where the signs on the door indicated they had been closed since before the shop was offered. One required a phone call before the visit which I attempted but only got a "full mailbox" message--I'm probably not getting paid for that one. The other required no phone call before the shop and I worked my way through the report to get the automated system to accept a "This location was closed." repeated over and over. At first they kicked the report without telling me, but I spotted it and protested. It now shows I will be paid.
It is certainly not your fault if the ad was not present, but if contractually you would be paid only if the ad was there, then you got suckered. Welcome to Mystery Shopping!