Could not find 'UserSecretsIdAttribute' on assembly 'ef'

You have run into this issue: https://github.com/aspnet/Configuration/issues/543

The solution is to change your call .AddUserSecrets() to .AddUserSecrets(Assembly assembly)

See this announcement about how deprecating project.json required a breaking change to user secrets: https://github.com/aspnet/Announcements/issues/209