If you had all the beauty you desired, you could with boldness leave the wood and make your way among mankind.
Petrarch addresses this final section—called an envoi, or a congedo (or farewell)—not to Laura or to us, but to his little poem. How do we know that? Well, by reading Canzone125, of course. As you might have guessed, Canzone 125 comes… right before Canzone126 in the Canzoniere. You might also have guessed that it's a lament about being miserable in love—so miserable, in fact, that Petrarch can no longer manage a decent line of poetry. In the end, he disses his own poem, saying that it needs to keep its ugly face hidden:
O my poor verse, how rough you are! I think you know it: so stay here in this wood.
And now the congedo of Canzone126 starts to make more sense… sort of. Again, we have a conflicting set of translations. In addition to Mark Musa's interpretation of line 66 ("If you had all the beauty you desired"), we've got Durling's translation, which is closer in meaning to the Italian: "If you had as many beauties as you have desire, you could/ boldly leave the wood and go among people."
The difference? Musa's translation speaks only about the desire of the poem to be beautiful (i.e., if you had your way, you'd be beautiful). Durling's interpretation is more complex. His version seems to be saying that the poem itself is jam-packed with desire and wouldn't it be great if the poem had the beauty to match it?
The general purpose of an envoi or congedo is to allow the poet to have a final word about the poem itself—or to address a real or imagined person directly. The congedo for 126 kind of does both of those things. Petrarch addresses his poem as though it were personified, walking on two legs through the wooded banks of the river and into the wide world.
But don't be fooled. He's actually talking about (and to) a poem. These final three lines, then, is really a wish that his work can do something that he can't do just at the moment: move into the world (and succeed), away from the place where Laura once was.