To fix this for new skills going forward you will want to take the step mentioned in this post by cd'ing into this directory:
and running this command:
npm install pixi.js
That should keep this from happening for new skills.
For the skill you are currently seeing that error in you can either cd into the skill's top level directory and run
npm install pixi.js or if you have already followed the steps that I outlined under "to fix this for new skills..." above then you can also fix this for your current skill by removing the skill's
node_modules and running
npm install again.
Let me know if you continue to see this error after going through those steps.