When you add a Slack Admin card to a Flow for the first time, you'll be prompted to set up a connection for that channel. You will have to be a member that channel. Setting up a connection will allow you to connect the Flow to your Slack Admin account, save the data, and reuse that connection the next time you build a Flow with this card.
Note: You can create multiple connections and manage them from your Settings page.
To create a new connection from an Action card:
Click New Connection.
Add a Connection Nickname. This is useful if you plan to create multiple connections to share with your team.
Select a Slack account. If you are not already signed into a Slack account, you will be prompted to do so.
Click Authorize to grant access to your Slack data. Note that this allows the flow to send messages on your behalf.
You can reuse this connection any time that you use the Slack card in a Flow. You can also set up and save multiple connections to share them with your teammates.