What should I do about this situation?

So, I just started working at this restaurant two weeks ago, and I've been out of training for less than a week. I had a doctor’s appointment scheduled for 10:40 today, but I was also scheduled to work from 10 to 5. The schedule comes out Sunday evenings, and I was working all day yesterday until 8 PM, so I didn’t see that I was on the schedule until after that. When I realized I had the appointment, I couldn’t reschedule it. I texted the group chat to see if anyone could cover my shift, but got no response. I also texted the manager last night and didn’t hear back. This morning, I texted and called her, and she told me to reach out to people individually instead of the group. I did that, but still no responses. Eventually, I told her I couldn’t find anyone to cover, and she replied, ‘No ma’am, we will run the shift without you. This will be your first and only warning for not being able to work a scheduled shift.’ How is that fair? What should I do in this situation? How can they terminate a new hire when I did what I could to cover my shift?

How’s that?
What should I do about this situation?
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