Install Leave calendar in Site Collection
You must be a tenant administrator to install the Leave Calendar app. You must also have full rights to work in the App Library. The best way is to follow the steps below.
1. Log in to Office365 with the Tenant Global admin account.
2. Open the site collection where you want to install the Leave calendar app.
3. Click Settings and Add an app.
4. Click on SharePoint store (on the left side).
5. Search for Devcore.
6. Click on Leave Calendar icon and on Buy it in the next step.
7. Click Trust it.