The custom role needs to be assigned to the service account. A custom role is created using either of these procedures: Creating a Custom Role for GCP Organization Application or Creating a Custom Role for GCP Standalone Application.
Follow these steps to assign permissions to a service account:
Login to GCP Console using the administrative privileges.
Select either ORG level or PROJECT from the selector on the top. If a project is selected the following steps need to be repeated for all projects managed within Britive.
Select IAM & Admin -> IAM from the navigation menu.
Click ADD.
Enter the service account and custom role and click Save.
In case of projects/folders, repeat these steps for all projects/folders to be managed within Britive.