Can you please clarify what you mean with
it doesn’t even post
Do you mean it is not reachable via its IP address or are you trying to ssh into it?
My first approach would be to check the bios settings when you’ve a monitor connected. Maybe there is some kind of strange setting set there!?
2 things I found on a quick search:
1st Look if your GPU is actually designed to have a „headless“ start - if not you most likely need that dummy adapter that others mentioned
2nd In the bios settings look for something like an “Halt on all errors” option and disable it. If the GPU is the problem than you might get to start it anyways without any monitor connected