The first troubleshooting note on
https://www.maartenbaert.be/simplescreenrecorder/troubleshooting/ suggests:
"Some Linux distributions (e.g. Ubuntu 17.10) will by default use the Wayland display server. Since Wayland doesn't provide a standardized API for screen capturing, SSR can't support it (yet). The solution is to switch back to the X11 display server. On Ubuntu, this can be done by logging out, clicking the 'gear' icon on the login screen and selecting 'Ubuntu on Xorg', and logging back in."