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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's saying you're trying to use "replace" on an undefined variable. Look for where you're trying to use "replace," and make sure that you've defined the variable above that line and within its scope.