You might have had tried similar steps but can you try following these steps to see if it resolves the error? It has helped other affected users in the past:
- Log in to your Google Drive account from within an incognito/private browser window. This is important to ensure that you're connecting with the right account.
- Log in to your Slack account from within the same incognito/private browser window
- Go to https://my.slack.com/services/
connections - Find and click in to the Google entry.
- Click the Disconnect button.
- Go to the following page and re-authecticate your Google account:
https://slack.com/apps/A6NL8MJ6Q-google-drive?tab= more_info