Can I still ask my prospective advisor (guide) for a confirmation letter in the middle of Covid-19
It is fine to ask. Life goes on as best we can manage. The situation is worldwide, of course, and we don't yet see the end of it. 

In a letter, you could, if you wish, say you are sorry if the request is coming at a bad time and that you understand the global situation. 

You may not be able to get a reply in time, of course, or at all. But there is no problem asking. There is only a week left. 

And note that people will want to start up as normal again as soon as the situation allows. Things may be delayed.