How can I create a Scopus Author ID?
You don't have to. A Scopus ID is created automatically when your first publication is indexed in Scopus.
I'm a Master's degree program student, I'm working on my thesis and don't intend to write anything else. Is it mandatory for me under the Rector's Measure to create unique personal identifiers?
No, it isn't. According to Rector's Measure No. 28/2016 academics and teaching staff, doctoral degree students, and other authors who publish are obligated to create identifiers and the associated profiles. Publications are understood to mean journal articles, articles in conference anthologies, monographs published by specialized publishers, etc. Usually, such publications are funded from grant projects. If the result of, e.g. your GAUK investigation is an article written during your Master's degree program, create unique personal identifiers.