Registration Button not working on CiviCRM Event Page
CiviCRM 5.19 is not compatible with WP 5.6.   WP 5.5 had a change that required an update which is in CiviCRM 5.26 https://civicrm.org/blog/kcristiano/civicrm-526-and-wordpress-important-notice
I'd recommend CiviCRM 5.32.2 currently as that version has all the security fixes in 5.28 (latest security version) as well many other bug fixes/enhancements.
Please look at https://docs.civicrm.org/sysadmin/en/latest/upgrade/version-specific/#civicrm-529  as this further documents some of the follow up changes that affect Paths and URLs in WP