See: [FREEPBX-23002] Add support for Asterisk 19 - Sangoma Issue Tracker
install.php and installlib/installcommand.class.php have not been updated to allow installation of Asterisk 19.
Until they are, the following modifications are required prior to running ‘./install -n’:
sed -i s’|“19”;|“20”;|’ /usr/src/freepbx/amp_conf/htdocs/admin/libraries/utility.functions.php
sed -i s’|“19”, “ge”|“20”, “ge”|’ /usr/src/freepbx/installlib/installcommand.class.php
sed -i s’|“19”, “ge”|“20”, “ge”|’ /usr/src/freepbx/install.php