In the past, individual users were able to install adds-ons for Docs, Sheets, and Forms. Google has changed this so that the administrator now needs to whitelist add-ons before they can be installed.
Prior to installing an add-on, I suggest you go into the G Suite Marketplace settings area and allow visibility of add-ons/apps from the marketplace for your users:
Once that is done, this video shows you how to install add-ons for your users: