I can't say for sure and can't test it for now, but try executing the following commands from command line:

sudo snap install gnome-3-26-1604
sudo snap connect peek:gnome-3-26-1604 gnome-3-26-1604:gnome-3-26-1604

I currently would recommend to install Peek via command line. Peeks needs to be connected to the gnome-3-26-1604 snap, and snap does not yet fully automate this connection. The current status is that the second of above commands should be done automatically on installation, but installing gnome-3-26-1604 is still a manual step. I know the snap developers work on automating that, too, but it probably will take a few releases of snapd until that works.