'portfolio_format_leap2' is invalid name of the component. Because of 'portfolio_' prefix, Moodle expects it to be portfolio plugin called 'format_leap2' and AMOS created strings for it into '/portfolio/format_leap2/lang/en/portfolio_format_leap2.php'. Because formats are not really pluggable, their strings must be defined in the core subsytem file - /lang/en/portfolio.php in this case.

If you want, you could use leap2a prefix as in

$string['leap2a_missingfield'] = 'Required Leap2A entry field {$a} missing';