A message saying something cannot be done, without saying what you do about it, is a dead end. Eight of them are now a door.
What has changed
Every message now names three things: what is in the way, who or what that is, and the button that resolves it. A few examples:
| What you run into | What you get with it |
|---|---|
| The slot is taken | The appointment sitting there, open it straight away |
| The slot has been offered to a customer | The option to withdraw that offer |
| The staff member is not rostered | The button to open that day up |
| The receipt still has to be finished | The button to finish it |
What it does not do
The message does not go on to carry out the action for you once you have cleared the obstacle, with two exceptions where that is self-evident. Cancelling somebody else's appointment or withdrawing an offer to a customer are decisions, and something you decided yourself should not quietly happen again because a dialog closed.
If you are not allowed to do what the remedy asks yourself, because you are a staff member and changing the roster is not your role for instance, you see the explanation and not the button. A button that gives you an error is more annoying than no button.