Once you've created the Service Account, you need to explicitly give it access to your spreadsheet. If your spreadsheet is viewable by anyone with its link, no further action is needed. If this is not the case, then in the "Credentials" tab on the left side of your Google API Dashboard, copy the email address of the Service Account you just created. Then, in the Google sheets UI, click the "share" button and share the spreadsheet with the service account. The video below illustrates this process.