"Unknown power management support - can't continue"

When attempting a power operation on a host, Foreman will either use the compute resource (if it's a virtual machine) or BMC (i.e. IPMI) to power on/off the host.

Host is actually a VM on a compute resource

Foreman probably thinks it's "bare metal" and doesn't realise it's on a compute resource (i.e. built outside of Foreman). Navigate to the compute resource via Infrastructure > Compute resources and click the Associate VMs button. This will try and locate VMs on the compute resource that Foreman has host entries for (based on MAC or IP usually) and link them.

If Foreman linked them, the host in the UI should now show power/console buttons and appear to be on the compute resource.

Host is bare metal machine

BMC controls can be used to power a host on/off via IPMI. Edit the host and create a new BMC network interface, which should point to the management interface on the host, with a correct IP address and credentials. A smart proxy with the BMC feature enabled is required, this is the service that Foreman will contact to run IPMI commands through.