First steps with Craft Nitro: hostname not working after successful install
After cheating via discussion in the comments, I’m certain that your DB_SERVER is incorrectly set to 127.0.0.1 when it should in fact be mysql-5.6-3306.database.nitro.
In a perfect world this would have been set for you when Nitro edited .env, but in a pandemic-ravaged dystopia your next best option is to use whatever nitro context indicates.