Its very late, but i had this problem too…
For me the solution was enable the mod using:
a2enmod proxy_fcgi setenvif
a2enconf php7.4-fpm
After this restart apache
systemctl restart apache2
If do not work, try to purge the mod, re-install
apt purge php7.4-fpm
apt install php7.4-fpm
and if not enable automatcaly, manualy enable the mod using again:
a2enmod proxy_fcgi setenvif
a2enconf php7.4-fpm
After this restart apache and refresh the webpage