Server starts but is offline

Trying to check out some plugins, so I start my server up. 60 minutes of waiting, then it's online. When I go on Minecraft, it says the server is offline. I join and it says "outdated server". I refreshed the page, restarted Minecraft, restarted the server, turned it off and on 3 times. I don't know what to do now. Any help?

EDIT: Well now it starts but goes offline immediately. I've seen people have this problem, so I'll check out some other posts.

Did that. On 1.12.

Nevermind! It works! I just have to use direct connect (i don't get that at all) But it's all fixed! Thanks