First off, what size tank is it in and what tankmates are in the tank, if any?
At that size, bloodworms are a great staple. It is rare, but I’ve heard of a few eels that will eat pellets. Don’t be surprised if the eel goes on a hunger strike still. I’ve had one of mine that seemed to be acclimated as soon as I put it in the tank only to have it go extremely skiddish after a few days. My first two that I got, which were both 4”, ate bloodworms from day one. The third one that I got at a later date, at about 8”, ate the first night, but then refused to eat for over a week. I eventually got it eating by throwing a huge mix of about 8-10 different foods in the tank all at once and seeing what it liked, which ended up being bloodworms. Once you get it eating something, you can get some weight on it with that, and try to introduce new foods with the food it likes. I eventually got my eels to eat just about anything frozen, and live night crawlers and red wigglers. My eels won’t touch pellets though. I don’t mind that because I can put pellets in for my catfish and not worry about the eels eating everything right away. My three eels
At 4”, I recommend feeding bloodworms, if it will continue taking them, and then try to work in some cut up red wigglers. Try to cut the red wigglers into pieces about the same length as the bloodworms and mix them in with them. The eel might eat them out of excitement and then get a taste for them.
If your eel has eaten flakes, and you want to get it on pellets, you can definitely try to get some smaller carnivore pellets for it. If it starts eating pellets then you should be able to upgrade the pellet size as it grows.
Has your eel eaten anything in your tank yet? Can you confirm that it has eaten flakes?