Because "Intro to Rigging" is structured to start simple and gradually add on more rigging components, the earlier simple ones are more challenging to complete the full-body rigging required for Motion Library. I recommend going through the rigging part of OkaySamurai's Motion Library tutorial video: https://www.youtube.com/watch?v=ZTevkEadskk&t=1091s
To be specific about what is missing with this 03_sidesample, here are the absolutely mandatory steps:
- Add handles and tag them with body tags (Neck, Waist, Left/Right Wrists, Elbows, Shoulders, Hips, Knees, Heels, and Toes)
- Make "Left Leg" and "Right Leg" layers crowned (independent)
- Adjust the positioning of "Left Arm," "Right Arm," "Left Leg," and "Right Leg" layer origins
- Add Limb IK, Body, and Motion Library behaviors
Additionally, for a better-looking result, you will want to
- Put stick handles through rigid parts of arms, legs, and feet
- Limb IK: uncheck "Auto Arm Bend" and check "Reverse Arm Bend Left" (as this is a strictly left-facing character)
- Adjust Motion Library's orientation
Here is my version for your reference: https://shared-assets.adobe.com/link/8c95f8cf-25d8-4a54-45db-fa7b4ebe4e6f
Good luck!