You should upgrade your DirectAdmin. This is an error in their provided Let’s Encrypt ACME client. I.e.: the agreement URL is hardcoded, but not up to date.
See also: https://help.directadmin.com/item.php?id=638
The letsencrypt.sh file should have been updated automatically by “CustomBuild” if you’re running DA 1.51 or newer: https://www.directadmin.com/features.php?id=1867 Perhaps your DA version is 1.50 or older? Check out the “See also” link above on how to upgrade it manually.