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.
Taming a unicorn is the same as a equus meaning a passive tame. You will need rockarrot or meoberries if you cant get a hold of rockarrots. Or troodon kibble if you can. All you need to do is look at the effectiveness button once riding it and once it stops and does a little noise feed it with your rockarrot or mejoberry. Also be careful because they will kick u off or since they run while you are riding and taming, they might run into rexes, Raptors, carnos, or allos.