Just about all retail DVDs are encrypted.
You will need a program to decrypt them for you or you can't copy them.
Another problem is most retail DVDs are DVD9 (9gig dual layer) and connot be copied strait to a DVD5 (4.7gig single layer DVD-R) without a program to shrink it.
There are lots of programs out there to do what your trying to do but I recommend using two programs from Slysoft.
CloneDVD2 allows you to shrink a DVD9 disc to fit on a DVD5 disc any AnyDVD decrypts the retail DVD on the fly.
Go to Slysoft.com and check them out.
Good luck!