So you are openning Gazebo the robot simulator on Ubuntu 18.04 Bionic, ROS Melodic.
But you see this problem in red in the terminal:
[Err] [REST.cc:205] Error in REST request
libcurl: (51) SSL: no alternative certificate subject name matches target host name ‘api.ignitionfuel.org’
You will still open Gazebo fine. But how can I get rid of this?
Fix Gazebo error