Why can’t I add a related video to my YouTube Short?

Usually it’s an eligibility or video-status thing, not a Short-specific bug. Open YouTube Studio on a computer, go to Settings > Channel > Feature eligibility, and check that advanced features are enabled. YouTube requires advanced feature access for a related video. Then edit the Short in Studio and choose a video from your own channel that’s public or unlisted and follows the Community Guidelines. Private videos, videos from another channel, or a channel without the needed access won’t qualify. If you’ve just completed verification, check the Feature eligibility page again before retrying; that page is where YouTube shows what your channel currently has enabled.