You can set up Slack notifications in your Salesforce flow, but there is a surprisingly easy point to get stuck on. That is to set the action element of the Slack notification to the “Immediate” side.
Most people may wonder what “immediate action” means! you may be thinking, “What the heck is “immediate execution”? But it’s something that everyone is definitely looking at.

Yes, I have. I know you have seen it lol. Well, most people would unconsciously press the plus button under “Immediate” and add an action element, but if you do that, the Slack notification will not work properly. Instead, you need to create a separate branch of the Slack notification and add the action element to that branch.
Another branch? You thought that, didn’t you? I thought so too. To set up a Slack notification, you must first click here to create another branch.
The “Add Scheduled Path (Optional)” is simply a feature that allows you to do things like execute a process one hour after a trigger is pulled. (This is the function of a record trigger.)
For example, after setting up a trigger as shown below, click on “Add Scheduled Path (optional)”.

Then, as shown below, the next element is set to be executed as soon as the trigger is pulled, just as in “immediate execution”. Since it is “0 minutes later,” it means there is no waiting time.

Then, a “Slack” branch will be created as shown below, and you can add an action element to that branch and set up Slack notifications to be successfully notified.

I hope this is helpful, as many people unexpectedly get stuck here when setting up Slack notifications!