I think it is best to migrate the whole server to a new operating system and Plesk. There were different issues with AWStats paths in the past that were related to missing repositories which resulted in problems after updating. As CentOS 6 is long end of life and Onyx 17.8 is also no longer maintained I guess that an automatic update was attempted, failed and left AWStats in a state where is is simply missing a component. But just guessing.
The easiest and best solution is to migrate using the Plesk Migrator extension. An upgrade to Obsidian 18.x cannot be done, because CentOS 6 is not supported. A dist-upgrade of CentOS is not supported by CentOS, so the remaining option is the migration.