Slack resource does not exist or has not been granted access

I’m trying to connect a rule on slack integration on sentry. We are running on premise, and I’ve configured all needed for the integration.
I’ve created a private channel, and added the designated sentry app (in slack) to the channel.
When I try to create a new rule - I’m getting this error - although I’ve granted all the access needed.
Who can help in this scenraio?
This support link wasn’t helpful (I’ve followed the suggestions here in vain)

Someone knows what’s the issue here? I’ve tried anything I can thing of.

Anyone?