On this page, you will find more information about the most common causes and most relevant solutions for the MercadoLibre error '400 - The field 'to.user_id' is required'. Do you need help straight away? Visit our support page.
The Mercado Libre, Inc. documentation indicates that error code '400 - The field 'to.user_id' is required' is caused by the following problem: "Message without receiver (need to add "to")".
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