@Ali Sanhaji - Welcome to Microsoft Q&A and thanks for reaching out to us.
When you are trying to create the subscription, your default billing account is the M365 billing account. You can't create Azure subscription under it. So, in the Create subscription window, you need to manually select your Azure billing account. Afterwards, you should be able to select an Azure plan.
Hope this helps. and please feel free to reach out if you have any further questions.
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