I'm trying to write my literature review but there are hardly any published papers related to my work. What should I do?

If you think that a literature review needs to contain as large number of references, then this is a problem. But it's not about the number of references, it's about reflecting the state of the field as it is.

If there aren't many papers to write about, this in itself is a worthwhile observation about the research in your field. Assuming you have good reasons why your research problem is important, a lack of pre-existing literature can help to justify your own work.

Also, if not much has been done already then it is far easier to make a significant contribution because the most basic questions haven't yet been answered.

So it's not necessarily a huge problem if there aren't many papers to work with, provided you have good reasons why other academics should be interested in your work.

Go into detail on what is there

One advantage of having only a small number of sources is that you can be a bit more comprehensive, going into more detail about each paper. This just isn't possible when there are thousands of articles (when you have to be much more selective).

Because you have fewer articles to choose from, you may end up citing lower-quality sources. If this is the case then it is OK to point out limitations in the existing research and use these as justification for further work.

Check the "cited by" information

If you have a few relevant papers, check to see who else has cited them. Maybe you wont find anything useful, but maybe you will find newer relevant articles that you might not have found through a keyword search.

You can also look up the authors of the papers you have to see what else they have published.

Broaden the search

Can you relate your work to a broader problem? Does it contribute in some small way to something larger? If you have a small number of articles in your niche, how do they frame their research in their introductions?

If there really are no papers to work with...

It is unusual for a research project to be completely unrelated to anything existing. Most research takes place on the edge of existing knowledge, not in an isolated void.

But if, somehow, you come up with an idea that is totally new, you have to think about who is going to be interested in it. The value of your work will be judged by other academics, whether in the form of a thesis defence or peer-review, so you need to find some way to relate it to problems other academics are interested in.

See also

How to find a gap in the literature

‍How to write a compelling literature review