How do I update an old project?
I came across this issue myself when starting my first Drupal 8 project.
This is most likely due to the version of Drush you are using being not compatible with Drupal 7. You can check your Drush version by running drush --version in the command line.
If Drush 9 or above, you should downgrade to version 8, i.e. composer require drush/drush:^8.
Tip: if ever wondering which Drush (local or global) you are using, type which drush in the command line.