Without knowing more about the installation environment, it's difficult to say exactly what's causing this issue.
We strongly recommend following the recommended installation instructions - this may be a feature incompatibility issue between your chosen MariaDB version and AtoM 2.6, which expects MySQL 8.x. There are also a few PHP 7.3 incompatibilities we have fixed in the upcoming 2.7 release, but which may still exist in the current AtoM 2.6 releases, which recommend PHP 7.2.
For step by step installation instructions, please see:
Cheers,