As an admin, login to G-Suite console (https://admin.google.com/u/2/ac/home) - you should see something like this:
Open security settings (hamburger menu - security - settings) - you should see something like this
Click API permissions
Click Trusted apps
You should see something like this:
Click on yellow + to add Zapflow as trusted app
Select web application as application type
Enter Zapflow application OAuth2 Client ID as
and click Add button.
Now everything should be set.