Due to policy in some companies, individual user cannot install apps from MS Store. |
You can see for example message like this
Sorry, Microsoft 365 has been configured to prevent individual acquisition of Office add-ins
We do not support not authorized installation, however with help from your administrators you can still install apps in alternative way.
Preferred way by using Admin Center - https://docs.microsoft.com/en-us/office/dev/add-ins/publish/centralized-deployment
Using SharePoint App Catalog - https://docs.microsoft.com/en-us/office/dev/add-ins/publish/publish-task-pane-and-content-add-ins-to-an-add-in-catalog (which have few flaws like: disabled commands, no Mac support and in hybrid ecosystem Admin Center is advised to be used).
Unfortunately, for Office Store (unlike for example on a Windows 10 apps Marketplace) there is no way to have completely offline installation (beside development mode – sideloading, which shall not be used for production environments)