first off, welcome.
As its a HP you may well have a locked bios, i.e. it will only accept certain chips. Hence the message you saw.
I recently had a similar issue i was fault finding with, it turned out it was the mobo.
Have you altered the machine in any way? have you added/changed the GPU? In the case i'm thining of, the power supply has been exceeded due to various upgrades.
What processor did you try it with?