reposting it here
Maybe this is the issue. I think in that version this export endpoint was removed (It will be deprecated, but in the future, in the latest version it is back). You need to use the GraphQL Admin endpoint tho.
Use cURL or some GraphQL UI (e.g. Insomnia REST Client) and run this mutation.
mutation {
export(input: {format: "rdf"}) {
response {
code
message
}
}
}