File /usr/local/directadmin/plugins/custombuild/admin/build does not exist. The following command may fix the problem:
gcc -std=gnu99 -B/usr/bin -o /usr/local/directadmin/plugins/custombuild/admin/build /usr/local/directadmin/plugins/custombuild/admin/build.c && chown root:admin /usr/local/directadmin/plugins/custombuild/admin/build && chmod 4755 /usr/local/directadmin/plugins/custombuild/admin/build