Due to policy in some companies, individual user cannot install apps from MS Store.
We do not support not authorized installation, however with help from your administrators you can still install apps in alternative way.
Admin Center / SharePoint App Catalog
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).
It turns out that for Office Store (unlike for example on a Windows 10 pps Marketplace) there is no way to have completely offline installation (beside development mode – sideloading, which shall not be used for production environments)