Upgrading your Inn may require more SP than you can currently store.
To increase the total amount of SP you can store in your Heavenhold, you will need to upgrade your Sky Garden.
By increasing your SP Storage Capacity you will be able to upgrade your Inn, which is necessary to purchase new buildings and upgrade them to higher levels.
Once you acquire the new Heavenhold (added with Season 3), you can further expand the storage capacity by upgrading certain buildings. Like this: