1. First you need to make sure that the Sheets API is enabled - in the screenshot, the link is just above the highlighted one
2. Make sure the Google API project is selected, go to the control panel
https://console.cloud.google.com/apis/Click Credentials
3. Click Manage Service Accounts
4. Depends on whether there is an existing one or not
If there is, go to it, click next on the Keys tab
If not, create.
5. To create a file, click Create new key
the received file and you need to upload it to the integration settings along with email and Google API key