When a Microsoft Office 365 user tries to install a new app for Microsoft Outlook Web App on the Installed App page, the New icon to add an app from the Microsoft Office Marketplace is not available. Therefore, the user can't add the app.
The My Marketplace Apps role is not enabled in the role-assignment policy that's assigned to the user.
Note By default, the name of the role assignment policy is "Default Role Assignment Policy."
Update the role assignment policy to enable the My Marketplace Apps role for the user. To do this, follow these steps:
Sign in to Office 365 as an admin.
Click Admin, and then click Exchange.
Click permissions, and then click user roles.
Double-click Default Role Assignment Policy, or double-click the custom policy that's assigned to the user (as appropriate for your situation).
Select the My Marketplace Apps check box, and then click save.
When you set roles in a role assignment policy, you should remember that the change affects all users who are assigned that policy. Admins may want to create custom user role assignment policies that have different roles to assign to different users.
For more information about role-assignment policies, see Understanding Role Based Access Control.
Still need help? Go to Microsoft Community.