If you want to connect your BigQuery account for Data transfer manually (using service account connection) you need to follow these steps:
Go to your Google cloud, and find IAM & Admin section on left side menu
From there, go to IAM section
Click Grant access button
Add whatagraph-etl@whatagraph.iam.gserviceaccount.com as new principal and assign BigQuery Data Editor and BigQuery Job User roles.
Click Save.
Also you can watch this video with all the steps mentioned above.