Install the package in the sandbox with the sandbox install link, then connect a separate Mailchimp test account. If the sandbox was copied from a production org that was already connected, clear that copied connection before you reconnect.
Clear a connection copied from production
If the package post-copy script was selected when the sandbox was created or refreshed, the copied connection is cleared for you. If it was not, the next sync can also disconnect when the stored org id no longer matches the sandbox. If the old connection is still there, clear it yourself.
1. Open Execute Anonymous.
In the sandbox, open the Developer Console, then Debug, then Open Execute Anonymous Window.
2. Run the reset.
Run mone.McAPI.resetData();
This disconnects every Mailchimp account and deletes the Mailchimp data stored in that org: accounts, audiences, members, campaigns and email activity. Only run it in a sandbox. Never run it in production.
3. Connect again.
Open Mailchimp Settings and connect a test Mailchimp account.
Note: A sandbox connected to your live Mailchimp account can change live subscribers. Field mappings, the Data Wizard and Flow actions in the sandbox all write to the Mailchimp account you connect.
expired_token
If connecting returns OAuth token validation failed: expired_token, the token exchange with Mailchimp did not complete, so nothing was saved. This usually happens when the return to Salesforce was not completed within about 30 seconds of approving the connection in Mailchimp. Open General Settings and connect again, completing the Salesforce login step promptly. If you are sent to a Salesforce login page or see a URL error on that step, see I am having trouble connecting my Mailchimp Account, what can I do?
Package Not Found
That message comes from the install link, not from a setting inside the package. Use the sandbox install link while you are logged into the sandbox, as a user who can install packages.
