Hi @Developer GZ
This is because your tenant does not have an MS 365 subscription. When you try to access MS 365 services, you will need an MS 365 license.
Before doing so, please purchase or start a free trial of an MS 365 subscription for your tenant. Then, go to the MS 365 Admin Center to assign the license to the user on whose behalf you are sending the mail.
Hope this helps.
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