Hi Shivam,
The topic "/gazebo/ModelState" contains only one "Pose" field, while the topic "/gazebo/ModelStates" that you are using contains more than one "Pose" field. When you see fields end with "_SL_Info", that means the corresponding field contians more than one element. As shown on the screenshot below, there is a "Pose_SL_Info", which means Pose is a variable-size field.
As the error message mentioned, you would need to use a "Selector" block to pick one "Pose" field from the variable Pose arrary before you tried to extract "Position.X", this is equivalent to specify the index of the Pose array.
Hope this helps,
Josh
