I prepared a course, but cancelled it due to lack of participants. How to describe in CV?

I'm pretty sure this isn't the answer you want, but I don't think you should mention it in your CV. You have no proof of the quality of the materials you prepared (since they were never used or assessed and you got no feedback on them), or even that they exist (unless you have uploaded them to a public repository or something; even then you are unlikely to get much credit for them).

Where they might come in handy is during an interview: if teaching is a significant part of the job (particularly teaching in a similar field), you may be asked 'how would you prepare a course' or something, in which case you can describe your experiences - although this relies pretty heavily on you being asked the right question at interview, and evidence of practical experience of actual teaching, and positive feedback from participants, will be viewed more highly if you have it from other projects.

Don't put this on your CV. It suggests that you think preparing for a single meeting and writing a course outline is some kind of big deal that is worthy of praise; it isn't. If you'd prepared a significant amount of the teaching material, that might be different. It also draws attention to a failure: the natural interpretation is that you thought that your course would be interesting to the students and at an appropriate level for them, whereas they disagreed.

I would include something generic that describes your experience as one of the duties of your position, such as:

Contributed to course design and preparation.

This highlights the experience you gained (which is the main benefit, I think). If you are asked about it in interview, you can talk about that specific course and what you did. In the context of a conversation, it will be easier to explain that the course didn't go ahead without it reflecting negatively on you.