Only white screen when booting on Mac

Hello,
I have a MacBook Pro from 2012 with 16 gigs of RAM and 1 TB SSD storage.

I’m trying to installing Ubuntu 2.

But I can’t start Ubuntu after installing it.
I created a boot stick with Balina Etcher and I installed Ubuntu without any problems.

But when I try to start Ubuntu I can only see a white screen without any command prompt or Grub.