You cannot prevent, in the date picker control, that the user select a date outside a certain date range (you can only limit the date picker based on the year). Please consider creating a new feature request in the PowerApps Ideas board for this scenario.
What you can do is to show a visual indication to the user that the selection is incorrect. In the example below, I added a label under the start / end date pickers and changed the border color of the pickers to indicate an error:

You can accomplish that by setting the following properties (assuming the start / end date picker controls are called respectively datePickerStart / datePickerEnd, and the label that shows the error is called labelInvalidDates):
datePickerStart.BorderColor: If(datePickerEnd.SelectedDate < datePickerStart.SelectedDate, Color.Red, RGBA(101, 128, 187, 1))
datePickerEnd.BorderColor: If(datePickerEnd.SelectedDate < datePickerStart.SelectedDate, Color.Red, RGBA(101, 128, 187, 1))
labelInvalidDates.Visible: datePickerEnd.SelectedDate < datePickerStart.SelectedDate
If the selection is fixed, then the label will disappear, and the border of the date picker controls will return to its default value:

Hope this helps!