As written in your original error, the problem you have is in the format of the contents parameter. Here is how you send it:
And here’s where you can find how FB expects to get it: Standard Parameters - Conversions API - Documentation - Meta for Developers
A list of JSON objects that contain the product IDs associated with the event plus information about the products. Available fields:
id,quantity,item_price,delivery_category.
Also, there is usually not any point in sending items to FB, it is the format for GA4. For FB, you send item data in ‘content_ids’ and/or ‘contents’ params.