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.
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1215238/image.jpeg)
Hope this helps,
Josh