It's the issue with the 0.18 Lemmy version. Hopefully it gets resolved with the new version.
Only 0.0.35 will work with 0.18 due to some breaking changes. It should be available on Google Play now.
The user needs to make a comment/post and then you click ...
for more options on that comment/post and one of them will be Appoint as mod
.
@KinkyRachel
@lemmynsfw.com