Take a look at the item again:
You might think (A)
that a floor-length dress is conservative
(B) until you see the
one that Christina wears. (C)
A psychedelic swirl of pinks and purples that attracts everyone's
attention.
You wanted to do this:
You might think that a floor-length dress is conservative.
Until you see the one that Christina wears. A psychedelic swirl of
pinks and purples that attracts everyone's attention.
Adding a period after conservative makes a fragment. Until you see the one that Christina wears is a subordinate clause, so it cannot stand alone as a complete sentence.
You might want to review the rules.
Go back to the sentence to try again.