If your php.ini file exists then in configuration file simply uncomment the extension:
;extension=php_pdo_mysql.dll
Else follow below steps
step1:php -v
step2: Install php mysql extension
php 7.0 sudo apt-get install php7.0-mysql
php 7.1 sudo apt-get install php7.1-mysql
php 7.2 sudo apt-get install php7.2-mysql
php 7.3 sudo apt-get install php7.3-mysql
step3: service apache2 restart
step4: php artisan migrate