Could not op <username>

Open up MC, log in successfully
Start server, able to join successfully
In console/ MC Server GUI, I type "op bmangaman"
Returns with "Could not op bmangaman"

Tried to manually add myself to ops.json - is wiped when I start server by running "minecraft_server.1.12.2"

Works when not in online mode (changing online-mode=false in server.json)

I try to view my username in browser by going to: https://api.mojang.com/users/profiles/minecraft/bmangaman - does not load/ work