In the video tutorial there's also a quick passage that shows the same fix, Here is the video at the relevant time step:
Common ErrorsHere are some common errors that users have reported and how to fix them.
Equip Unequip Ability has a Category with ID 3636…. that cannot be foundThis errors happens if you setup a new character with built-in abilities preconfigured from the Character Controller Demo.
The easiest way to fix it is to go through each Equip/Unequip Ability, within the Character Locomotion script under Item Abilities, and re-selecting the item set category to force the category ID to be serialized.