Let's say you have a 24-hour hourly rental.
1. Create a new reaction.
2. Enter the name.
3. Select the type - Error and enter the message text about the minimum duration.
4. Click Add condition.
Pick a Duration condition.
Specify the number of steps so that the message is displayed when booking is 1 hour long or less.
Apply the condition and save the reaction.
Check the result on the widget. Try to book only 1 hour: