How to OP yourself on a Minecraft Vanilla server?

I started a Minecraft Server but every time I try OPing my self it never works, I've even tried entering it on the OPs list in the server Folder I don't know if I entered it wrong on the OP list but I need help!

Guest

You need to open the Minecraft server. You see the console. Below is a typing area where you type /op your name then in Minecraft it'll say you've been opped. Hope this works

Anna