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.