Hi, Good question!
A couple of follow-ups.
Have you checked that there are actual resource status changes? What filters are you using for Status and Severity? This is to validate that there are valid events that should be sending notifications. For more information: https://docs.aws.amazon.com/elasticbeanstalk/latest/dg/AWSHowTo.eventbridge.html
Have you validated that the OAuth Token is setup: https://api.slack.com/authentication/oauth-v2
It looks like you've seen this page, but https://docs.aws.amazon.com/eventbridge/latest/userguide/eb-api-destination-partners.html has links to Slack documentation for help as well.
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