He had a spread. The short leg was assigned, the long leg cancels it out but it doesn't happen instantaneously. So he temporarily was on the hook for -300 $AMZN shares or whatever, which made his buying power massively negative. Then the long leg gets executed and it all evens out in the end for a relatively small profit or loss (the delta of the strikes), but that can be a nerve-wracking weekend if you don't understand the process.

Robinhood's UI is not particularly informative in this process either.