If it won’t allow you to feed it, knock it out and let it wake up then try again. Most likely due to the fact someone else tried to tame it before. Also, it tells you when to feed it at the bottom corner of your screen.
On The Island I found one on Herbivore island, if u don’t know where it is google it. When I found it I was panicking because I didn’t have any crops or berries so I went on my raft home got the crops went back and tamed it. They spawn on Herbivore Island a lot. If your on The Island always look there first. :)