"The turtles are yours."
Translation:De schildpadden zijn van jullie.
I'm not sure what you mean exactly. But a few pointers:
- the conjugated verb comes in second position (after the subject de schildpadden in this case), more on Dutch word order: https://www.duolingo.com/comment/3733010
- in the sentence van jullie is the possesive pronoun (what yours is in the English sentence), more on possessive pronouns in Dutch: https://www.duolingo.com/comment/3732923
Because it's just an isolated sentence, there's nothing in the exercise that indicates whether the "you" is plural or singular.
So Duolingo accepts the singular variants as well.
"De schildpadden zijn van jou" worked for me, for instance.
And "De schildpadden zijn van u" should work as well.