Hi @Abhilasha G T ,

I tried your problem in my PDI please check below code 

Create onChange Client script in Priority field and add below code 

function onChange(control, oldValue, newValue, isLoading, isTemplate) {
    if (isLoading || newValue === '') {
        return;
    }


    // Get the planned start and end date fields
    var plannedStartDate = g_form.getValue('planned_start_date');
    var plannedEndDate = g_form.getValue('planned_end_date');

    var startDate = new Date(plannedStartDate);
    var endDate = new Date(plannedEndDate);
    var currentDate = new Date();

    if (newValue == '1') {
        if (startDate > currentDate || endDate > currentDate) {
            alert("Please select any date in past");
            g_form.clearValue('planned_start_date', '');
            g_form.clearValue('planned_end_date', '');
        }
    }

    // Condition for Priority 2, 3, or 4
    if (newValue == '2' || newValue == '3' || newValue == '4') {
        if (startDate < currentDate || endDate < currentDate) {
            alert("Please select any date in future");
            g_form.clearValue('planned_start_date', '');
            g_form.clearValue('planned_end_date', '');
        }
    }
}

Please mark my answer correct and helpful if this works for you

Thanks and Regards 

Sarthak