After importing the Entities package into Unity 2020.3.17f1, I get the following compiler error:
Library\PackageCache\com.unity.entities@0.17.0-preview.42\Unity.Entities\Serialization\BinarySerialization.cs(47,81): error CS8377: The type 'T' must be a non-nullable value type, along with all fields at any level of nesting, in order to use it as parameter 'T' in the generic type or method 'NativeList<T>'
I believe you’re using the updated collections package which the older entities package doesn’t support yet
Downgrade your collections to com.unity.collections@0.15.0-preview.21
downgrading the collections package doesn’t work (when I click the “update to 0.15.0-preview.21” button, it reimports things but doesn’t downgrade the package), maybe because others depend on it?
Turn on show dependencies in options and it’ll show dependencies. It’s probably the jobs package.