Take a look at the item again:
To make his long shift at Bernie's Burger Emporium
tolerable, (A)
Kelvin used ketchup, (B)
to make smiley faces on the burgers
(C) that he assembled
at the grill.
You wanted to do this:
To make his long shift at Bernie's Burger Emporium
tolerable. Kelvin used ketchup, to make
smiley faces on the burgers that he assembled at the grill.
A period after tolerable makes a fragment. To make his long shift at Bernie's Burger Emporium tolerable is an infinitive phrase. No phrase can stand alone as a complete sentence.
You might want to review the rules.
Go back to the sentence to try again.