Reproduction steps:
1. Open user's attached project "ggj20-pop"
2. In the Project window, go to Samples/2D Animation/3.1.0/Samples/4 Character (or Samples/2D Animation/4.1.0/Samples/4 Character)
3. Open scene "_Character"

Expected result: No prefabs are missing or user is informed of a needed package
Actual result: Prefabs are missing with no explanation on how to fix it

Reproducible with: 2019.3.0b11, 2019.3.0f6, 2020.1.0a22
Could not test with: 2017.4.36f1 (no Package Manager), 2018.4.17f1, 2019.2.20f1, 2019.3.0b10 (2D Animation Samples not implemented)

Reproducible with package versions: 3.1.0, 4.1.0
Could not test with package versions: 2.2.1-preview.2 (2D Animation Samples not implemented)

Workaround: Adding the 2D PSD Importer package

Note:
Not reproducible using the 2020.1 2D template because 2D PSD Importer package is installed by default
2 prefabs are also missing in Samples/2D Animation/3.1.0/Samples/5 SpriteSwap scene "1 Animated Swap"