You will need to give it the Manage products with Catalog API use case.
4. Add this application to your Business portfolio
5. Create a system users and give the system users access to both the application and the catalogue. This user will be required to get the access token for your app.
Click on Create On the next sceen click on Download your credentials and then click on done.
Add test users in google cloud API
in google cloud, navigate to audience and then scroll down to add users.
Add developers in Merchant Account
in Merchant Center, add a user in Setting > Access and services > add person (or manage an existing user) untick admin and make sure to to tick API developer.