Your Master's thesis should make a significant, novel contribution to the field. Your thesis hypothesis should address a research gap which you identify in the literature, a research question or problem that has not been answered in your research area of interest. This shows that you have developed expertise in the body of knowledge and theoretical issues in your chosen research area.
Step 1: Focus Your Research Area
Before you start trying to identify gaps in the literature, you need to figure out what your area of interest is, and then focus and narrow that research area. If you don't narrow down your initial research area of interest, you'll end up wanting to research everything. You'll overwhelm yourself with all the research gaps you find because there are still a lot of unanswered research questions out there.
Step 2: Read, Read, and then Read Some More
Read (a lot of) research articles: this is going to be time-demanding, but you really do need to read through a lot of research articles in your research area to become an expert in it. That being said, what you use from the articles that you read should relate directly back to your focused research questions and hypothesis. Don't waste your time getting sidetracked by issues that don't relate to your research questions and hypothesis.
Follow the research trails of seminal articles and authors using Web of Science and Scopus:
Read meta-analyses, literature reviews, and systematic reviews: these papers delve deep into the literature, examining the trends and changes over a long period of time in your research area and summaries of previous research findings.
Step 3: Map out the Literature:
Keep track of what the authors told you and the questions that occur to you whenever you read anything - an article, a book, a book chapter, a dissertation, etc. This will also help you write your thesis introduction later on and help you avoid unconscious plagiarism.
If you find don't find any answers to one of your questions, you've probably found a research gap from which you can develop a thesis hypothesis and experimental project. Get feedback from your advisors before you get too carried away, though!