the exact same thing happened to me this past week using internet restore on my 2015 MacBook Pro. Unavailable.
I tried various things but now I’m getting a black screen with a big symbol of something indicating it can’t proceed. So I’ve taken the advice today to purchase a USB with Carolina on it. Only thing is though - I’ve ordered a 2020 iMac, ETA January 13. Wish I’d known before about a USB. Good luck with your problem.