Check in BIOS Settings and make the IGPU/GFX/etc or Integrated Graphics is enabled for your motherboard.
also please post the Make & Model of your Motherboard and Windows and BIOS version installed.
In Device Manager when you have the GPU card installed are there any errors or Yellow flags showing under "Display"?
also with just the APU installed do you get any video Output when you first power up the computer?
Are you able to enter BIOS Settings with just the APU installed?