Fixed it. The issue was in the look up.

If(
!IsBlank(Dropdown_Employee_New.Selected) && Slider_Days.Value > 0,  // Ensure a valid employee and positive days

// Update the TotalLeave in EmployeeLeaveBalances
Patch(
    EmployeeLeaveBalances,
    LookUp(EmployeeLeaveBalances, EmployeeEmail = Dropdown_Employee_New.Selected.EmployeeEmail),
    {
        TotalLeave: LookUp(EmployeeLeaveBalances, EmployeeEmail = Dropdown_Employee_New.Selected.EmployeeEmail).TotalLeave 
                    - Slider_Days.Value  
    }
);