check these facts

1. check the generate graphql(.gql) file in the project root level.

2. if the the mutation is available in the .gql file go the play ground click Root in the breadcrumbs.(check the below image). In the roo both Queries and mutations available

3 if the mutaion is not available in .gql (notmaly at the bttom) file in the root level, try to delete it and re build the project