The CentOS 8 Packages are outdated since RH dropped the support for CentOS 8 in favour of CentOS Stream. You need a repository subscription to get the official enterprise packages or switch to a os for which icinga provides free packages like ubuntu/debian/opensuse/fedora/centos7
If you want to learn and get started with icinga I really recommend to try out ubuntu or fedora, after you get familiar with the icinga ecosystem you will find some bugs can report them, or even fix them. I’m mentioning that, because being part of the developer community has some benefits, including the developer subscription. You can use this to extend your knowledge by installing icinga2 on enterprise operating systems.
As I’m writing this about developers and so on, I reread the icinga developer subscription limitations and this might work for you already:
“The Developer Subscription is an offer for individuals and entities who use Icinga in small environments for development or production use” Icinga » Developer Subscription