There may be a problem with the reverse lockout which is a cable that runs from the shifter to the brake pedal. Altghough on many cars you can't take it out of park if this is disfuntional. Ther could be a problem with the shifter cable or the shifter linkage at the shifter or the tranmission. Check the linkage at the shifter and the transmission. Then there could be an internal problem with the reverse shaft. If that is the case the transmission will have to come aprat to be repaired.