If you downloaded Grav v2.0.3 + Admin, only Admin 2 is installed.
If you downloaded Grav v2.0.3 Core, you should install Admin 2 using bin/gpm install admin2
I can reproduce the issue when downloading Grav v2.0.3 Core and installing Admin (1) using bin/gpm install admin
I don't think Admin (1) is foreward compatible with Grav 2.0
I think the Get Started section on the download page is misleading since it states bin/gpm install admin. I'll create an issue on the Grav repo.