When you say that the author's purpose is confused in Robert Frost's "The Road Not Taken", in a sense you are confusing three different issues. The first is whether Frost himself was confused in some way, the second is whether you are confused by the poem, and the third is whether the text of the poem contains lines that its intended audience would find confusing.
As to what was going on in Frost's mind as he wrote, or what his intentions were in writing, those are actually not a very important part of the reading experience of poetry, and also, unless we were capable of telepathy (which we are not) ultimately undecidable.
As for your confusion, part of it may be due to seeking a singular "purpose" in a poem. Poems are like songs in that they are intended to provide aesthetic pleasure, and perhaps to stimulate reflection, not to make an argument or convince you of anything. The basic theme of the poem is presented quite clearly.
Frost's poem describes the experience of a narrator having to make a choice. He is hiking or riding in the woods and has a choice of two paths. He can only follow one of them. He chooses the more desolate of the two. He reflects on the fact that he could return at some other time and explore the path he did not choose, but predicts that he will not do so.
On a metaphorical level, this addresses how we all make choices in our lives, and the choice to do one thing means choosing not to do something else.