Hello and welcome to the community!
The StarterGame project does not have all the supporting files needed for the Jack player slice. (I’m not sure why, but I’ll pass this observation on to the team that supports this sample.)
There are two ways you can solve this problem so that you can start using the Jack character:
Option 1: If you want to keep the StarterGame as your default project, you’ll need to copy the AnimationSamples directory from dev\SamplesProject over to dev\StarterGame so that the StarterGame project has all the asset files it needs.
Option 2: Using Project Configurator, switch your default project to SamplesProject.
Alternatively, you could use one of the supported player slices that comes with StarterGame. Since StarterGame assets are now in a Gem, you’ll need to look in the Gems folder. For example, in Asset Browser, look under Gems -> StarterGame -> Characters -> Assets -> Valena -> Slices to find the Valena player.