Hello there,
The JSON permission certificate is how Google Cloud Provider allows third parties like Terra authenticate and upload files to GCP GCS - it is best practice to create a service account with only the necessary principals and create a certificate . To generate the certificate, open your GCP project and in the sidebar, navigate to [IAM and Admin > Service accounts > Select an Account > Keys] and hit the button that says + Add Key then choose JSON. Then upload this certificate on the Terra dashboard; if the certificate is valid, the modal will close without outputting an error message.