Your description says your packages are based on ROS melodic, but actually your franka_ros package forked from franka https://github.com/erdalpekel/franka_ros seems pulled from kinetic. I wonder if there will be conflict in such way.
Another problem is, I didn't find the file franka_description/robots/panda.transmission.xacro in your franka_ros package as your description.
Your description says your packages are based on ROS melodic, but actually your franka_ros package forked from franka https://github.com/erdalpekel/franka_ros seems pulled from kinetic. I wonder if there will be conflict in such way.
Another problem is, I didn't find the file franka_description/robots/panda.transmission.xacro in your franka_ros package as your description.