Fix with Rector

what does it means “fix with rector” ? what shall I do to fix this errors exactly ?
Can you please help me to instal this drupal-rector to fix issues automatically ?

Hi Elena

Rector is an automated tool which can handle most changes necessary to ensure Drupal 9 compatibility on outdated modules. In general it is typically easier to simply update the module in question (assuming it has had a release supporting Drupal 9) as you then need not learn Drupal Rector. In your case, the Full Calendar View Plugin does have a release supporting Drupal 9: fullcalendar_view 8.x-2.8 | Note the machine name fullcalendar_view matching the path highlighted by the Upgrade Status report. As such, you should be able to download, unzip, and install this module and thereby ensure compatibility.