Not all types of papers are publishable
Many papers produced in the course of undergraduate studies and research projects don't meet the criteria to be published in an academic journal - and not because of writing quality (which can be fixed), but due to the type, scope and domain of that research. It may well be that whatever research work is described in that paper doesn't fit the basic criteria of novel, unpublished research results, and so it won't be publishable no matter how well you write about it. You can write an good paper about experiments reproducing some well-known result - that wouldn't be publishable. You can write a survey paper of some topic that'd get a great grade, but that wouldn't be publishable unless it's in some meaningful way superior than the currently best published survey.
It may well be that some undergraduate research project does contain publishable results, I've seen multiple such projects, but most of them don't, and would not be publishable (no matter how well written) without substantial further research/experiments/etc. I have no idea whether your results on "the screening and hit prioritisation of anti-cancer drugs for repurposing use against malaria" constitute publishable research results, but your mentor is the person who should know best whether your research constitutes something that's considered publishable according to the standards accepted in your field, and if they have said that it's not, then I'd trust them.