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.