JSON:API error unprocessable entity status 422 entity_id this value should not be null
Check if you have the JSON:API Extras module enabled.
If you do, you or someone else may have disabled the field for the resource you are trying to POST to at /admin/config/services/jsonapi/resource_types.
In this case, I had disabled the entity_id field for my flag resource, causing the &quot;null&quot; error even when I posted a value with axios.