Run php --ini
and notice file path on Loaded Configuration File
.
Then run command like cat -n /etc/php/7.2/cli/php.ini | grep intl
to find if the extension is commented or not.
Then update loaded configuration file by commenting line by adding ;
such as ;extension=intl
This can happen when you install php-intl
package and also enable the same extension on php.ini
file.