My macbook won't let me update it

Hello I need help! I've been trying to work on this all day, for context I have a macbook pro early 2015 that is stuck on the el capitan update. I have tried and googled vigorously on how to get it to update, when I got to apple's sight I tried to update it to the newest version that my mac goes up to and it says that I can't. Then I tried to do the next one which is the sierra one and when I try to update it it says that It has to be the 10.12 when the sierra version I was downloading was that version.