In PX4 release/1.16, I get “unknown target” if I try to run make px4_sitl gz_x500or any other vehicle like gz_standard_vtol. I’ve tried running make clean and make distclean but that didn’t fix it. However, those targets work in the previous PX4 versions.
Make sure you have the proper gazebo installed from the install script
Figured it out, I had to delete the px4_sitl_default build folder.