I have a turtle and he won't eat lettuce?

I feed him lettuce 2 times a week and ever since I got him he would bite it and would spit it out.
Does he not like it?
Ugh I'm confused
Update: He is a Red Eared Slider Turtle and they told me he is a juvenile.
Update 2: If I try to feed him ground beef would it have to be cooked or raw?
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