When taming put the food in the last hotbar, do not press the hotbar key as you yourself will eat the food. In order to tame crouch walk behind the unicorn then it will give you a ride option, once you are riding the unicorn the unicorn might buckle that is when you press the feed button(NOT the food hotbar button) for example of Xbox it is Y button. After you’ve fed the unicorn t will run around for awhile wait until it bucks again and feed it once more. Continue that process until tamed.
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.