The answer that Steve gave covers it. There were probably a limited number of spaces "available" and unfortunately, your response came after the slots for that start had been filled. There should be contact information included in the email that you received. Call/email them to verify that was the case. If there is any information on when next openings might take place, they will let you know. In meantime, be vigilant in looking for that next opening to be sure that you get in before that group is filled
What Steve said
The opportunity may have reached capacity. If it did send and email and explain what happened. You may have to wait for the next opportunity to pick.
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