Hi @FullMoonLTD , welcome to the community forums. Thank you for reaching out with your question!
It sounds like you are using the new abandoned cart/checkout email flow. With this setup Shopify email is not able to send an email to a customer who is not already a subscriber to marketing emails, as noted in the error message you received. You can read more about why an abandoned cart email may not be sent in our Help Center here: Shopify Help Center | Opt in to the new abandoned checkout automation.