@Boyang_Wang
I've come across this exact same issue, and I'm working in Enterprise v10.9.1: within the 'New' Map Viewer, the button for building custom arcade labeling expressions only appears for Feature Services. It's simply not there when you try to do the same for Map Image Services. The documentation "Configure labels (Map Viewer)" fails to mention this distinction. Yet, it is possible to create custom label for either service type within 'Classic Map Viewer'.
The Helpdesk was able to confirm the same behavior, and is hopefully going to check with the development team to see if this was in the plans for future versions of Enterprise. I'm not sure if it has been addressed as off v11.0/11.1.