Per my test, setting validation on start time and end time dropdown fields by using Power Apps is not available.
As a workaround, you could use list validation in the list settings.
List validation: =IF([start time]<[end time],TRUE,FALSE)
- Create start time and end time dropdown fields in the list.
- Go to List settings -> Validation settings -> List validation: =IF([start time]<[end time],TRUE,FALSE)
- If the start time is greater than the end time, error message will show in the form and you cannot submit the form.
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