Since drive.file scope only lets your app interact with files that have been created by the app or explicitly shared with the app, you need to manually share the two sheets with your app’s service account.

Go to Google Drive->Locate both spreadsheets->Share them with your app's service account email (found in the Google Cloud Console under IAM -> Service Accounts)->Grant Editor access for the sheet you want to write to and Viewer access for the one you want to read from