Short answer: The problem affects new clean installs on 5.4/5.5 but shouldn't affect upgraded sites. To quickly fix a new site, you can run cv as follows:
cv ev 'CRM_Upgrade_Incremental_php_FiveFour::addActivityDefaultAssigneeOptions();'
Long answer: As Jitendra pointed out, the change stems from #11998, which updated both the code/UI and the database. Of course, the PR went through some testing and passed. What gives? The testing focused on taking an existing database (perhaps a real site; perhaps just a local dev site) and applying the change on top. The PR worked because it defined an upgrade procedure for the database. However, in a clean-install, the database doesn't use the upgrade procedure. As a quick-fix, you can manually update the database. As longer term, we should issue a new version which automatically updates the database.