Seems like only admins and global moderators can change post owners right now.
So if so, you might be able to choose these settings on the permissions screen?
Or even if the button does not show to anyone who does not have this permission.
I noticed that @Ben-Wang made a post inside this thread, which was subsequently deleted. However on the forum homepage, the recent posts still shows the content & a link to the deleted post. I'm sure this would be solved when soft deleting is tackled, but just wanted to make you aware of it in case it happens anywhere else.
Thanks for reporting https://github.com/designcreateplay/NodeBB/issues/1366