Both Unity & DOTS/ECS Packages are undergoing rapid changes.
Please use exactly the recommended Unity version & Packages:
Unity: 2020.3 LTS (!)
Do not upgrade yourself. DOTS is in preview, newer versions always break something. Let me worry about this for you, only update when I update 😉To get started, import DOTSNET from the Asset Store, open one of the example scenes & press Play!