I am applying for PhD programs in the USA, one of the schools requested a PhD research interest. The problem is I do not know how to write it, and I didn't done any research in my masters degree, are there any solution?
For the abstract just state you did a masters without a thesis. Universities are used to the fact that not all applicant followed a standard model for their prior studies.
As for research interest I’m unclear what the challenge is? I would pick a topic that is not too general (cell biology) so that the interest is hard to elaborate on or too specific (Actin-related protein 2/3 complex subunit 1B) as to suggest hyper-specialization. Make sure you pick a topic that can be studied in the program you are applying to and explain why it is interesting to you.
You need, at least, to name a subfield of your major field. Not mathematics, say, but functional analysis or something. Narrow is good, but not so narrow that it would be hard to find an advisor. Hopefully you have given this some thought.
If you have any ideas about specific problems in that area it will be a plus, but not everyone has those on entry and needs to discuss with (potential) advisors before getting started.
For someone with just a bachelors degree, less would be required. Having a masters they will expect a bit more. But not a detailed research plan.