'./update_dependencies.sh'     error

When i was adding dependencies with ubuntu 18.04 and ros melodic, it has this error code :  
' ERROR: the following packages/stacks could not have their rosdep keys resolved
to system dependencies:
fssim: Cannot locate rosdep definition for [fssim-xacro]
fssim_gazebo_plugins: Cannot locate rosdep definition for [fssim-pcl-ros]'
how can i solve it? Thank you.
 
