That error is referenced here: Unified Namespace Generator — OXID eShop developer documentation 6.0.0 documentation
Possibly something went wrong during namespace generation, there are some steps listed in the document to resolve it. (It is not the file which is not found but the class)
With your local install: i could see the error when using composer 1, you can upgrade to v2 with
composer self-update --2
then run
composer update --no-dev
In the shop root directory, set config.inc.php writable and run setup again by opening setup directory in browser.