That’s really clear feedback. Right now the horizontal splitter blends into the background, making it almost invisible. A simple fix would be to increase its thickness and give it a contrasting color so it stands out against the tab’s background. That way users can immediately recognize it as a divider and interact with it without confusion.

If you’re submitting this to the design team, you could phrase it like:

“In the Add Products tab, please enlarge the horizontal splitter bar and use a contrasting color. The current grey‑on‑grey design makes it hard to notice or use.”