The official MariaDB RPM repository (https://mirror.mariadb.org/yum/), from which CustomBuild downloads, has already removed version 10.3, therefore if this version of MariaDB is selected from Custombuild, when we do the build you show the corresponding errors:
This version will have to be removed from Custombuild or modified so that the RPM packages for 10.3 are downloaded from another repository.
Code:
Updating MariaDB
download_with_cache: downloading 'https://mirror.mariadb.org/yum/10.3/centos/8/x86_64/rpms/MariaDB-client-10.3.39-1.el8.x86_64.rpm' to '/usr/local/directadmin/custombuild/cache/MariaDB-client-10.3.39-1.el8.x86_64.rpm'
#=#=#
curl: (22) The requested URL returned error: 404 Not Found
safe_download: downloading 'https://mirror.mariadb.org/yum/10.3/centos/8/x86_64/rpms/MariaDB-client-10.3.39-1.el8.x86_64.rpm' to '/usr/local/directadmin/custombuild/tmp/tmp.FNxXNLDmiB.safe_download' failed (1/3)
#=#=#
curl: (22) The requested URL returned error: 404 Not Found
safe_download: downloading 'https://mirror.mariadb.org/yum/10.3/centos/8/x86_64/rpms/MariaDB-client-10.3.39-1.el8.x86_64.rpm' to '/usr/local/directadmin/custombuild/tmp/tmp.FNxXNLDmiB.safe_download' failed (2/3)
#=#=#
curl: (22) The requested URL returned error: 404 Not Found
safe_download: downloading 'https://mirror.mariadb.org/yum/10.3/centos/8/x86_64/rpms/MariaDB-client-10.3.39-1.el8.x86_64.rpm' to '/usr/local/directadmin/custombuild/tmp/tmp.FNxXNLDmiB.safe_download' failed (3/3)
This version will have to be removed from Custombuild or modified so that the RPM packages for 10.3 are downloaded from another repository.