There is no apache2-mpm-worker package because the mpm_worker module is included by default when you install Apache on Ubuntu's newer versions. It is contained in the apache2-bin package that is a dependency of apache2:

sudo a2dismod prefork event php* && sudo a2enmod worker

If you see any errors like cannot enable module (similar), then let me know with the whole output. Make sure you enable php-fpm when you use prefork, if you wish to use PHP.