Variable is never used in operation for dynamic inner parameter

[kumararvind]

Getting Error: {"errors":[{"message":"Variable "$userId" is never used in operation "listUserStudies".","locations":[{"line":2,"column":28}],"path":[]}]}

Query: { query listUsersStudies($userId: String!){ listUsersStudies(limit: 250, filter: { pk: { matchPhrasePrefix: $userId } }){ items{ pk sk status versionStr versionStatus } } } }

Variable : "{"userId":"d97ca8de9436"}

Its been 2 days I got stuck here, I tried many ways but nothing works.