Hi Tai,
It appears to me you're trying to assign "fields.name" inside a "sensor_msgs/PointCloud2" message using a "Bus Assignment" block. That doesn’t work because "fields" is an array of buses, and Simulink doesn’t support assigning to subfields of elements within an array using the "Bus Assignment" block.
As per the documentation (here), to assign values to elements of a bus in an array of buses, you should use a MATLAB Function block. So you’ll need to build the full "fields" array using a MATLAB Function block and assign it in one go.
Refer to the following documentation, to know more about assigning into elements of Array of Buses:
