European Civic Center Notifys Of A Friends Visit But The Never Goes Away. Help.

Weeks ago, the European Civic Center indicated that a neighbour had visited.  The notification has never gone away!  I have clicked on "okay", tried sending the neighbour a "thank you", but nothing will get rid of the yellow exclamation point.  Until that yellow exclamation is gone, I can't upgrade since no green arrow is available.  I have been able to upgrade the other areas of the European Mall, just not the Civic Center.  What can be done?