To be able to install the Outlook Integration, you must have particular versions of Outlook on your device with a Microsoft account. Below is the full list of versions and requirements.
Here is a list of the requirements to help you know if you can set up the integration and below you'll find a list of MS subscriptions:
To enable connected experiences with your Outlook if your Addins setting has been removed.
In Outlook Settings
Go to File -> Options --> General
Click the "Privacy Settings" button.
If the "Enable optional connected experiences" button is not checked, the 'Get Add-Ins' button (among other functions) will be greyed out. Check it.
Restart Outlook and the "Get Add-Ins" button should be visible now.