Each building upgrade has requirements (tap on the arrow button to view).
If you can not upgrade building right now, there are two possible reasons:
1 - You need to reach the required XP level. Once you reach it, the XP shield will be marked with a green tick as in the image below.
2 - You need to collect 24 stars while playing levels. Once you collect all required stars, the counter will be marked with a green tick as in the image below.
When you complete both goals, the upgrade window will appear after tapping on the arrow button.