Welcome to the Apple Support Communities!
It sounds like you may have had the iTunes Store app open on your iPhone or computer when these "service not available" pop-ups were happening. If that was the case, the easiest thing to do would be to quit the app or sign out of the iTunes Store. If you were on a computer, you could also check the System Status page at Apple to find out if a particular service is temporarily unavailable.
If these pop-ups were happening while you were surfing the web in Safari, then look at the resource below:
Stop pop-up ads in Safari
If neither of these scenarios address your issue, please reply with a little more information (what device? what application were you in ? etc.) and we'll be happy to help.
Enjoy your evening!
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