Some organizations restriction Google Workspace Marketplace installs from employees without express permission from their IT departments. To successfully unblock the installation you can follow these steps.
Allow List
An admin can simply allow the Salesroom Google Calendar AddOn to be installed by specific employees in the Google Workspace Marketplace Apps Settings.
Admin Install
An admin can simply click “Install App” in the Google Workspace Marketplace Apps Settings, find the Salesroom Google Calendar AddOn and install it on your behalf.