Before being able to deploy and use SharePoint Apps with SharePoint Online, you have to do some configuration.
Login to the Office 365 administration portal with an account with administrator rights and go to the SharePoint Online Administration portal
Then, once logged on the SharePoint Administration Portal, click on the Apps link available on the left
The first to do is to create an App Catalog by clicking on the Apps Catalog link and then choose to create a new catalog
This will redirect you to the site collection creation page, which will be automatically set to create a SharePoint App Catalog; just fill the “classic” fields – Title, adminitrator… and quotas
Then, once the site collection has been created, you can come back to the Apps section to manage your SharePoint App settings such as allowing (or not) end user to use the SharePoint Marketplace (Configure Store Settings), purchase apps – including free ones (Purchase Apps)
When purchasing an App (from a site or from the SharePoint Administration Center) you may get an error
“Sorry, we couldn’t determine which set of apps to show you. Please choose an available language from the options in the upper right corner.”
This is a pretty simple error which requires from you to change the language from the top right drop down menu