It's currently not possible to install ITRS OP5 Monitor on EL8 (Enterprise Linux, short for RHEL/CentOS). For an up-to-date compatibility matrix, please see this part of the documentation:
When will support be added?
The big reason why we have not yet launched support for EL8 is compatibility with our current WebUI: Ninja. Ninja is currently only supported on PHP5, and EL8 is shipping with PHP7. We are currently hard at work developing our shiny new WebUI, which - among a lot of other things - will allow for installation on EL8.
In other words: we are working on it, but it's not in the short-term pipeline and we recommend all users of OP5 Monitor 8 to install it on EL7.
We also recommend all current users of EL6 to update to EL7 now instead of waiting for a release of OP5 Monitor that is compatible with EL8.