First of all a disclaimer - it's impossible to diagnose with such sparse information over the internet. All I can offer is what I do in such a situation where a chicken is lethargic, inappetite, and watery poop and I've ruled out coccidiosis and egg binding. In fact, just last month I had two two-year old hens with just these symptoms. I treated them with an antibiotic and each fully recovered.
I keep an assortment of antibiotics on hand for such a situation. I try not to use the same antibiotic over and over for the same chicken, but a good all-purpose antibiotic is amoxicillin. You can usually find it without a prescription at pet stores or feed stores that sell aquarium supplies under the term "fish mox" or "aqua mox". Or order it here.
https://www.kvsupply.com/item/aqua-mox-250mg-capsules-100-count/P06184/ The dose is 250mg once a day for seven to ten days.
You haven't included your location, so I can't tell you if you will be able to obtain this if your state laws prohibit it. California and Oregon have laws forbidding companies shipping to you if you live in those states.