For WordPress 5.5.1 the minimum CiviCRM version is 5.26.2. I'd recommend 5.28.4 as that is the security release.
Due to changes in WP you may also have to update your settings.
For reference: https://civicrm.org/blog/kcristiano/civicrm-526-and-wordpress-important-notice
Even though the changes discussed come into effect in 5.29 this doc: https://docs....
I just came across this page, with a similar issue.