The first start is slower than every start after it. The server has to download its jar, generate the world and write its configuration files, and joining too early looks exactly like a broken server.
What to do
- Press Start and watch the console
Do not judge anything by the panel status alone. The console is where the server actually tells you what it is doing.
- Wait for the world to generate
The first boot creates the spawn area from scratch. On a fresh world this takes longer than any later start, and the console goes quiet while it works. That silence is normal.
- Look for the line that says it is ready
The server prints
Done (12.345s)! For help, type "help". Until that line appears, connection attempts will be refused.
The console shows the Done line and the console prompt accepts commands. Only then will the server accept players.
If the first boot stops instead of finishing, the usual reason is the EULA. The console will say so plainly.