The issue is apparently not theme-related.
1. Make sure the option “Allow users to re-post activity from activity stream” is checked in Settings menu > BuddyPress > Options tab > Activity Settings section (should be enabled by default). Developer’s site: https://bhargavb.wordpress.com/2020/05/23/buddypress-re-post-activity-wordpress-plugin-2/
2. Important: if BuddyPress is network-activated, then you need to network-activate the plugin as well for the option to show up in the network settings and the button to work.
3. The re-post button only seems to show up on users and groups activity updates (direct updates on their profiles), not the other kinds of activity posts (site new post, comment, new friendship, group join, etc.), unfortunately.