The mobile app and mobile web Wikipedia interfaces automatically pulls item descriptions from Wikidata, where an item description in the same language exists; the mobile app also allows editing of the Wikidata item description.
But when trying to edit the Cantonese item descriptions using the mobile app, an error occurs and the edit cannot be saved. The error message seems to suggest the problem has to do with the fact that the domain name (zh-yue.*) doesn't match the language code (yue). (The domain name vs ISO 639 discrepancy is discussed on T10217.)
Until T10217 gets fixed (it has already taken 9 years and is still open), please resolve this mismatch by letting zh-yue.wikipedia entries match up with the yue language descriptions on Wikidata for both reading and editing.