For some, apparently very intermittent reason, after an upgrade, the Administrative permission 'Can manage extensions' is not available. The database query

SELECT * FROM `phpbb_acl_options` WHERE `auth_option` LIKE '%extension%';

returns no rows.

This seems to be a very intermittent problem, but has been reported on a number of upgraded boards. Examples are: https://www.phpbb.com/community/viewtopic.php?f=556&t=2419806; https://www.phpbb.com/community/viewtopic.php?f=556&t=2417926 and possibly https://www.phpbb.com/community/viewtopic.php?p=14702946#p14702946