Schema.org for an online tutorial (JSON-LD)

Question

Google never validates my Schema.org structured data about online tutorials.

Features:
Price: free
available in French and English
No session (always accessible)

Here is my current code:

{
    "@context": "https://schema.org/",
    "@type": "course",
    "name":"Standard apertures of diaphram",
    "description": ".......",
    "availableLanguage":["fr","en"],
    "provider": {"@type": "EducationalOrganization", "name": "Alain Pré"},
    "offers": {
        "@type": "AggregateOffer",
        "priceCurrency": "EUR",
        "lowPrice": "0"
    },
    "hasCourseInstance": {
        "@type": "CourseInstance",
        "courseMode": "online"
    }
}

Can anyone give me an example of Schema.org structured data in JSON-LD to characterise an online tutorial?