To be able to add the "Organization Policy Administrator" role to your principal, ensure that you are currently selecting the organization resource first before editing/adding a role to your principal email:

1. Go to your Google cloud console page.

2. Click the project picker located at the top left of the page and select your domain on the drop-down list.

3. Go to the “All” tab then select the organization resource.

4. Proceed on adding the "Organization Policy Administrator" role to your principal.

Below is a sample where the organization resource is selected. You should see a domain icon next to it.