Hello @This_isYE, please update your question your topic! I think something like “How do I fix the warning “counter.display without context is deprecated””? should do.
The fix is simple, you use a context expression!
#context appendix_counter.display("A.")
P.S.: don’t hesitate to provide a reproducible example, for example, your code is missing the variable appendix_counter. Removing any unnecessary parts of your code to produce a minimal example also helps.