How do I allow users to install plugins from the Office Store (AppSource)?

If you are getting an error message like "Sorry, Microsoft 365 has been configured to prevent individual acquisition of Office Store add-ins." when trying to install the MailMerge365 plugin you need to login as an Administrator (or contact your IT department/administrator) to enable plugins as follows:

Login in to the admin portal and go to "Settings" > "Org Settings" (direct link: https://admin.microsoft.com/Adminportal/Home#/Settings/Services)

Find and click on the list entry "User owned apps and services".

A sidebar will appear with two check boxes. To allow installation from the Office Store, the first checkbox needs to be checked: