This message you are getting is just a warning to let you know that the ads may have already expired, and to avoid having you click on an ad, say 10 minutes after that ad had been assigned to you, just to receive an error that the ad is no longer available.
Most ads are allocated to an account for up to 1 hour. You don't really have to worry much about that warning but some ads to expire faster.
All you need to do is to refresh the page or click on the link under that warning box to refresh your allocated ads. When an advertiser purchases for example 1000 clicks those clicks do expire and can't be allocated to someones account forever. But refreshing the page ensures that those ads keep assigned to you.