Check if php7.3-json and php7.3-sqlite3 are installed
dpkg -l php7.3-json
dpkg -l php7.3-sqlite3
If they are, reinstall them
sudo apt install --reinstall php7.3-json php7.3-sqlite3
And below one should enable the mods and create appropriate symlinks:
sudo phpenmod json sqlite3
And post outcome for below one just to be sure ?
php -m
after enabling the two mods, you most always have to restart the web daemon to apply:
sudo service lighttpd restart
(taken from SqLite3 and json_encode() fault - #12 by deHakkelaar, thanks to @deHakkelaar)