You cannot combine VALUES() and SELECT - they're pretty exclusive to one and other.
Code:
INSERT INTO tblNotAvail(RTime, RDate)
SELECT tblTimes.RTime
, strDate
FROM tblTimes
LEFT
JOIN (
SELECT RTime
FROM tblRequests
WHERE Rdate = strDate
) As tblRequests
ON tblRequests.RTime = tblTimes.RTime
WHERE tblRequests.RTime IS NULL
P.S. you know what SQL Injection is, right?