After test, I think the problem is your response.

You should response like:

{
  "update": {
    "message": "Updated!",
    "props": {
      "attachments": [
        {
          "text": "Ready?",
          "actions": [
            {
              "name": "Go!",
              "integration": {
                "url": "https://somewhere.com?foo=bar",
                "context": {
                  "action": "event_submitted"
                }
              }
            }
          ]
        }
      ]
    }
  },
  "ephemeral_text": "You updated the post!"
}

And you may want to check https://developers.mattermost.com/integrate/plugins/interactive-messages/