You can dynamically send a message to a person in Slack, as long as you keep track of the person's Slack ID in Airtable. (It might also work with their Slack email address as well.)
But I don't think that you can use Airtable's User field for this. (You might be able to use the User field, but I'm not sure.) You would most likely need to create a new table in Airtable where you keep track of each person's name and Slack ID and Slack email address.
Then, in your "main table" in Airtable, you would create a linked record field that links to that new table of people that you just created.
Choose the person from the linked record field, and create a lookup field that brings in their Slack ID from the people table. You can also create another lookup field that brings in their Slack email address from the people table.
Then, in your automation, you can refer to that Slack ID field, which will be different for each person.
Here's the key step, though: Be sure to click on the little tiny "gear icon" all the way to the right of the "Channel/User" field in your automation to be able to dynamically insert the Slack ID (or possibly the Slack email address, if that works).
That gear icon is what enables you to insert dynamic values.
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