A problem I have had several times now. If I get an updates notification for Pages & Numbers in the App Store, the "sign in" box will appear, and when I sign in, it vanishes, only to return & request again & again & again. Any other updates available can be run, no problem. Signing out & signing back in does not work either. The only way I have found to "trick" it into working is to sign into our other App Store account which is for another Country, & then try & run the update(s). The box appears again, on trying to sign in it duly says this sign in is for another Country App Store, flips to there, I sign in again, & THEN it works! (Signing out, going over to the other App Store, signing in, signing out & back to the original Store does NOT work!!) This only happens on my iMac that was updated to Sierra. My two laptops that came with Sierra do not have this problem. There are a few people commenting over on the Apple Community website that have exactly this problem - to date Apples' answer is to uninstall the App, & re-install, but I don't really consider that a fix!! Any thoughts / other solutions?