I am interested to work and improve on one of the research papers which published on https://www.casact.org/pubs/forum/ but I found out that, research paper published on Casualty Actuarial Society, CAS, E-forum is a non-refereed and not peer-reviewed by any of the CAS committee as stated in the E-forum. So, is it okay for me to choose a non-refereed research paper as my Master's thesis topic ??
This is a question for your advisor to answer. But in general, yes, there is no reason why you can't take ideas from anywhere you find them. You should be pretty sure, however, that if you want to accept the conclusions of the paper as your starting point that you can establish their correctness for yourself.
In fact, even if the paper is a poor one, it may still contain an idea or two worth pursuing.
Also, you don't know why it doesn't appear in a peer-reviewed place. It is possible that it is intended by its author(s) as work in progress to be followed up later.
But, first, ask you advisor.