Your question: Why is 又 used there and not 再 in the above sentence?
First, you need to know the difference of 再 and 又.
1) 再 means“again”,can be used to repeat the same action in the future which haven’t happened yet;
2) 又 means “again” too, but is used for the past to emphasize the event had already happened.
For example:
服务员,再来一杯啤酒。
When I finished a cup of beer, I want to order one more, I can say “Waiter, one more, please.” So here I should use “再”。
老师,可以再说一次吗?
The teacher had asked me a question, but she spoke very fast and I didn’t get it, here I can say “Teacher, could you please say it again?” The teacher haven’t said it again yet, So here is “再”。
你怎么又迟到了?
Yesterday I was late, my boss was a little bit angry, and today I was late again, he asked me “Why did you late again?” I was late, this event had happened this morning, so here we should use “又”。
我这次又没通过考试。
Last time I didn’t pass the exam, and this time I failed again. I got my report and I knew I didn’t pass the exam either. So here we use “又”。
So you need to use "又" in this dialogue.
A: 现在恢复得怎么样了?
B: 没问题了,又可以爬山了。
If you want to know more about how to use "再" and "又"?Please check the free video.