When you use Service Account, it’s like you are creating a separated account for the application using the service (in this case, a separated account for the robot).
So, if you want to allow the robot to access a Google Sheets document from your personal account, then you need to share that document with the Service Account’s email. To do that, you need to go to the document, click on “Share” (upper right corner) and then input the email of the Service Account you created.
A sample showing this in the same post I linked before (GSuite - Gmail Activity Unauthorized client error - #12 by Mateus_Cruz).
Also, please note that some services, like Gmail, won’t work with Service Account unless you have domain-wide access enabled. The reason for that is that, although you can share documents or calendars with the Service Account, you cannot share your email account. However, if you have domain-wide access enabled, then the Service Account will be able to impersonate you and use your Gmail account.