Resolved! "php -m | grep sqlite" result was empty. I guess "sudo apt-get install sqlite php-sqlite3" didn't work. I used "php -v" to determine my php version (PHP 5.6.40-30+ubuntu18.04.1+deb.sury.org+1 (cli)), then did:
sudo apt-get install php5.6-sqlite3
service apache2 restart
service fop2 restart
php -m | grep sqlite
now shows:
pdo_sqlite
sqlite3
Now I can use the FOP2 Manager!