Installing Civicrm on Drupal 7 site
Look at the error log file of the webserver (probably apache2). If you see an entry like PHP Warning:  POST Content-Length of 21426352 bytes exceeds the limit of 8388608 bytes in Unknown on line 0
you will have to adapt the /etc/php/7.2/apache2/php.ini file in order to rise the value of two parameters to v.gr. 32M: 

upload_max_filesize = 32M
post_max_size = 32M


Don't forget to restart apache after updating php.ini at the command line interface by issuing:

sudo service apache2 restart