You'd want to find a more in-depth topic for a master's thesis, Reddit scraping and sentiment analysis sounds more like an assignment. Ask your supervisor if they have a topic they're researching on, and if you can join. Look around if your university has example projects or even better, open projects. Look around past year's theses if you can continue working on any of them (hint: future works section) Once you find a topic you're interested in and is niche enough, it's still too broad so you have to filter it down to research questions, for which you have to start an in-depth research about the challenges of the topic and such.

Don't panic, there are many topics that need research. I'm starting my thesis in audio processing - health AI / speaker embeddings / impaired speech / diagnosis assistance and it's wild west over here, partially because the data is not publicly accessible though