Migration website to Drupal 8 from version 7 is very different process as is used to be. The older upgrade process from one version to another was made over exited system, Drupal's files and data base were changing (updated) on the way. The latest Drupal version, version number 8, require another upgrade procedure. First you need to make new, clean install of Drupal 8 system, then you import (migrate) website configuration and content. Good news that Drupal 9 comes with modules that make updated easy: Migrate, Migrate Drupal and Migrate Upgrade. Before you start the process make sure all the modules from old website have their Drupal 8 analogs. And be sure you have a back up, you need to make one, in case you do not have now and before and after the upgrade. And the best and safest way to migrate to Drupal 8 is to do it on a test website and Drupal 8 gives you that perfect opportunity.