A phrase is a group words. So is a sentence, but a sentence, unlike a phrase, will express a complete thought. Phrases are like the building blocks of sentences (clauses). In order to make a phrase into a clause, a subject and a verb must both appear.  The phrase could be used as either the subject or the verb (depending on the clause).  It could be neither too, which would make it a prepositional phrase.   

In order to take the phrase "road not taken" and make it into a sentence, it needs to express a complete thought with a subject and a verb.  You can keep it really simple and add "the" to the beginning of it, and then finish with a "be" verb.  

"The Road Not Taken' is a fun poem." 

Of course, that's a bit boring.  You could spice it up by adding a bunch of prepositional phrases.  "Over the hill, through the woods, and by the stream, there is a road not taken by many people, because it is full of trolls and ogres."