disposal discharge pipe is not long enough
No need to rebuild the entire assembly! It's a simple fix, you just need a couple parts from the plumbing shop. All you need is a short flanged tailpiece (see below) and a slip-joint elbow (see below).

Use the metal retainer and gasket that came with the disposer on the flanged tailpiece after you have cut it to fit. Insert the tailpiece into the elbow after it has been cut to fit... voila!