There are five possibilities: First, a different building is still being upgraded. You need to wait until it is finished, use gold to finish it instantly or cancel the upgrade.

Second, the building you would like to upgrade is currently fulfilling another task (crafting, researching, healing, questing). You need to wait until it is finished, use gold to finish it instantly or cancel the task.

Third, the building is the Marketplace. The Marketplace cannot be upgraded.

Fourth, you do not meet all requirements. If that is the case, you can still upgrade using gold or buy the needed requirements in the shop.

Fifth, you have reached the maximum level of the building. Most buildings' maximum level is 30, only the Watchtower's is 15. This will be indicated by a red level sign next to the building.