How can I open the console on an Ubuntu 18.04.1 Minecraft 1.12.2?

Thank you!

Putty?

I'm assuming you mean using a v / root server on the screen? If so:

Connect to your server via PuTTY and type "screen -x (screenname)" (without the "and without the parentheses.) If you do not know the screenname, enter" screen -ls "(without the"). Now all open screens are displayed.

If you still need help, e.g. When connecting to PuTTY, you can always add me to Discord (name: Entity303 # 2079) or Skype (name: endivie02).