versionvirtual Hi
1) Try add this in \administrator\components\com_dtregister\models\fields\currency.php file like seen in screenshot :
'MXN' => 'Mexican Peso',
2) Thereafter, You may try replace the file as suggested here - /administrator/components/com_dtregister/models/payoption.php
Would suggest to take backup of your current files before performing aforesaid steps, in case rollback required whilst facing any issues e.t.c