RHEL8 is supported starting with SCOM 2019 UR1 : https://learn.microsoft.com/en-us/system-center/scom/plan-supported-crossplat-os?view=sc-om-2019#red-hat-enterprise-linux-server-8-applicable-for-2019-ur1-and-later
SCOM 2019 requires at least Windows 2016 : https://learn.microsoft.com/en-us/system-center/scom/system-requirements?view=sc-om-2019#server-operating-system