Hello Manuel,
Unfortunately, our ability to offer support for CentOS installations is very limited - Artefactual does all its testing and development against Ubuntu LTS releases, as per our documentation.
However, you may be able to find support from other users in this forum, and from previous CentOS posts. For example, here is every post that has been labelled with both "CentOS and "installation":
Additionally, several users have shared CentOS installation guides with the community in the past, and we have tried to collect links to those we know of on our wiki, here:
There is not yet a guide we know of for CentOS 8 and AtoM 2.6, but the most recent guide listed there is for AtoM 2.5 on CentOS 8 - consulting it and comparing against your installation process may help you uncover steps you have missed or done differently.
Cheers,