Thanks for your feedback about your installation issue. I think it can help other users.
By the way, the default with Ubuntu 20.04 is now to use a socket (/run/php/php7.4-fpm.sock) instead of a TCP connection (127.0.0.1:9000).
Another solution is to edit your nginx configuration file and use this socket:
fastcgi_pass unix:/run/php/php7.4-fpm.sock;
Instead of the TCP connection:
fastcgi_pass 127.0.0.1:9000;