The list that the errored email is set for (per the email's Details page) is above 69 contacts. The email is also showing as initially sent to a few contacts as well, meaning you'd have less than 69 left between the combinations of your initial email and the errored email.
You'd need to send to a smaller list that won't put you over the 100 maximum, or you'd need to upgrade to a paid account for a larger send limit.
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