After the required time (6 days) passes, the building shows a "!" mark, but when clicking on it, the upgrade tab (up arrow) does not work, so there is no option to complete the upgrade.
I am having some other issues since the recent update (7.21 - The Land of Opportunity) so this might be another one introduced in this update.
I am having some other issues since the recent update (7.21 - The Land of Opportunity) so this might be another one introduced in this update.