You can't really by using Admin Tools 5.5.0 and PHP 7.3 for two reasons.
First, the error message you are referring to is a PHP 7.2 and later compatibility issue which was fixed in Admin Tools 4.3.2, released all the way back in 2017, three years ago.
Second, the error message comes from the old version of the PHP File Change Scanner which used the same engine as Akeeba Backup. This feature was rewritten from scratch for version 5.5.0 and no longer uses the backup engine. Therefore it can't and won't load it. In fact, the entire administrator/components/com_admintools/engine folder is no longer present.
At this point I'm sure you have a really old version of our extension. Delete the folders plugins/system/admintools and administrator/components/com_admintools to regain access to your site. Then install Admin Tools Professional 5.5.0 without trying to uninstall it beforehand to complete the upgrade to Admin Tools 5.5.0.
Nicholas K. Dionysopoulos
Lead Developer and Director
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