This case generally may happen when you have added some email id but they are having status as unconfirmed or unsubscribed.
If you have an old email list and some users have clicked on the unsubscribe button then please respect the same, who don't want to receive your email you shall not forcefully send such an email to them, as that may result in spam complaint issue.

If you have a new email list created then make sure that your email ids you have added or imported are having status as "confirmed" as if that is "unconfirmed" system will not permit you to send them emails.

Screenshot of the issue shared in point 2 as mentioned below:

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