Try a different browser and disable adblock.
My uBlock origin sometimes hides the sharing menu in Firefox, perhaps Brave has some kind of built-in blocker which blocks function based on certain keywords.
EDIT:
Can you not share anything in the web interface? Or is it just a certain folder?
Are you trying to share publicly, with a group or with a user?
Is it possible to share the exact folder you get an error message in the web interface for using the mobile client?
This issue is most likely very different from yours.
Finally: What do you see in the owncloud.log?