As the error says, you can't use the
+
operator to combine a string with a time value.
This answer[
^] on StackExchange has the solution:
SELECT
COUNT(*)
FROM
Reservations R
INNER JOIN StudentReservations S
ON R.ReservationID=S.ReservationID
WHERE
DATEADD(day, DATEDIFF(day, '19000101', R.ReservationDate), CAST(R.ReservationTime As datetime2(7))) < GETDATE()
And
s.UserID = 12
;