I had the EXACT same problem with my turtles. They were rescues and i had not seen them eat in 3-4 days. Lettuce, pellets, earthworms, nothing they would just turn around. Remember they might also be getting used to you.
Couple things first.
1.They NEED UVB light AND heat lamp/bulb. (if you dont have the lights yet, 30 mins a day, take her out into direct sunlight cpl time per day)
2.They need both lights on facing their basking area.
3.Basking area must be easy to reach and when on there they can be completely dry. (I use flat rocks i found outside make sure its sturdy though so it dosent fall on your turtle)
4.They need the water to be 73-78 degrees Fahrenheit. (the first two days i didnt hav a heat i would dump warm water in their tank til i felt it was room temperature)
if they dont have the appropriate habitat they go into hibernation mode and get lethargic and wont eat regardless of what you try to give them. so please make sure you get all this down as soon as possible.
Jello Shots. there are two recipes on that page one is the original and the other is a moderated one for turtles with shedding problems. my turtles have shedding problems so i did the moderated one. WORKED PERFECTLY.
then i got an idea, because my turtles even tho they ate the jello shots they were still not eating their lettuce or veggies.
i got this idea to come up with my own jello shot to get them to eat their lettuce and fruit and veggies.
although the main ingredient is their staple lettuce. there are a cpl fruits mixed in and other things that are not staple foods.
so dont feed them ONLY jello shots. this will help tho getting some nutrients in them and giving them a taste of diff foods.
once my turtles got the taste for it, they now eat lettuce and pellets on their own, no more jello shots

1 small head of green leaf lettuce
2 carrots
1 small head of red leaf lettuce
1 canned tuna in water
1/2 cup of pellets
1/3 of ground up cuttle bone. (it costs like 2$ at any pet store with bird sections. make sure you take the hard back side off it can get impacted in their intestines. youll be able to tell easily what i mean by the "hard side" of the cuttle bone itself)
you can put a little bit some krill also if you'd like to give it a stronger smell so shes attracted to it more.
puree in blender.
add 1 cup of boiling water and 2 packs of knox gelatin (no flavors)
blender again a cpl seconds
pour in container
let it sit in fridge til "jello like"
then place in freezer
cut a piece off and just dump in feeding container once frozen
when feeding to the turtle make sure its in a different container not her normal tank it gets SO MESSY the water even turns green from the veggies unfreezing. if its still frozen thats ok. they tend to like to chomp away at it and it unfreezes in the water pretty quickly
some ingredients are not staple foods so once this batch is gone and she has a taste of the foods try feeding her again normally.
my turtles eat normally now. this just helps getting nutrients in them and appeals their appetite quicker.
if you have anymore questions please feel free to send me a message or friend add me. i had this same problem and it worked so well. good luck