See no Entities package in package manager

[BackgroundMover]

Its intentional-

Visibility changes for preview packages in 2020.1

We recently went through our package list to identify preview packages that are further from the verification stage, just dependencies of other packages or not under active development. In Unity 2020.1 and later versions, these preview packages won’t be listed in the Package Manager. However, if you’ve already installed these preview packages, they won’t be removed – you can continue to use them, and they will still show available updates in the “in-project” filter of the Package Manager UI. Al…

but you can add the package without using the manifest, by doing Package Manager → add package from Git URL → com.unity.entities (no version # necessary)