1.In VS2015 and above, VS requires you to download corresponding framework when loading the project.

So at least you've installed the corresponding framework. For System.dll,System.core.dll...
, they are assemblies from .net framework. You can close the solution=>restart VS and reload the solution
to resolve the warnings about system.xxx assemblies. If the warning persists, try repairing your .net framework.
2.For EntityFramework.dll
and EntityFramework.SqlServer.dll
, they're coming from EntityFramework
package. Please check your project file(xx.csproj) and make sure there's statements like these:

Then navigate to the hintpath to check if the required assembly exists there. (solution dir/packages/xxx). If you restore the packages to other specific path, just modify the hintpath element and make VS can find those assemblies, the warning would go away.