A user will only engage with your product if it's rewarding to them. This can mean internal rewards ("I enjoy this / feel good") or external rewards ("now I can

A user will only engage with your product if it's rewarding to them. This can mean internal rewards ("I enjoy this / feel good") or external rewards ("now I can earn money"). You've identified the missing external rewards, and have reasons why this cannot be changed. That leaves only internal rewards. The fact that your users are not engaging is hard data showing that the experience of your training course is not rewarding enough. Stated more bluntly, your product isn't good enough. Fix that. You must stop thinking in terms of "retention", because this leads you down the path of tricks and dark patterns. Focus on making your core training product inherently compelling. This is likely to be more than one small thing... there are probably a hundred separate improvements that you need to make to polish your core offering.