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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
I got the same thing?!
You are trying to apply
.mapto something that isnil, and Ruby cannot do that. It's hard to say anything more. This can help: https://docs.codewars.com/training/troubleshooting/