DAX is simple, but it is not easy. Watch the video to learn why, and remember: Practice. The. Fundamentals.
Article: https://www.sqlbi.com/blog/alberto/2020/06/20/7-reasons-dax-is-not-easy/?aff=yt
How to learn DAX: https://www.sqlbi.com/guides/dax/?aff=yt
The definitive guide to DAX: https://www.sqlbi.com/books/the-definitive-guide-to-dax-2nd-edition/?aff=yt