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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.
It means you are trying to use a method on something which doesn't have it.
What language are you using? Im getting the same error and I am using Ruby. I believe it is because there are metacharacters in some of the input that appear to be throwing off your regexp. Im currently struggling with the same issue, and personally it just seems like these test cases are a bit much for the fundamentals that a 6kyu is testing.