Most existing packages, such as EF6, do not support asp.net Core 5; Due to significant differences, they must be updated manually. If you want to use EF6, you need to stick with asp.net 5 (not Core).
I encountered a number of problems with EF6 package manager commands, such as Add-Migration , in VS2015 CTP6; I believe that they are still working on updates. I was able to get them to work in the .Net 4.5 library that my .Net 5 libraries reference, but I found that sometimes I had to uninstall and install EF6 again and again to get the package manager commands to work.