How can I solve this?
"Slack channels in this team can only be linked to boards in an associated Trello team. To link --- associate its team, or move it to an already linked team."
Welcome to Community!
In order to link your Trello and Slack workspaces (previously known as teams), you need to go to your Trello workspace settings and click the "Add to Slack" button there.
Here's how to find that page:
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