Here is the item again:
To reach the countertop where Sara had momentarily left the tuna salad.
You correctly identified the fragment as an infinitive phrase. To reach, the infinitive that begins the phrase, is the clue.
To fix this fragment, you could add a subject and verb, modifying the fragment like this:
Barney, the family's Dalmatian, reached
the countertop where Sara had momentarily left
the tuna salad.
Or you could use Punctuation Rule 5:
Main clause + Ø + infinitive phrase.
The correction would look like this:
Barney, the family's Dalmatian, stood on his hind legs
to reach the countertop where Sara had momentarily
left the tuna salad.
Or you could use Punctuation Rule 6:
Infinitive phrase + , + main clause.
And the correction would look like this:
To reach the countertop where Sara had momentarily
left the tuna salad, Barney, the family's Dalmatian,
stood on his hind legs.
Enjoy the palace! It has a great swimming pool in back!