Need help on diagnosing issue

[AYDANN6ix9ine]

So to keep it short, I took an old 2011 HP Pavilion PC and upgraded its power supply and graphics card (GTX 950). I tested the graphics card in my personal system and things worked fine. But when I booted the whole new PC up, it would go the startup screen and just stay there with no response. I tried many things, like clearing CMOS, rearranging memory, but nothing worked until I removed the GPU. When I removed it, I used integrated graphics and everything was working normally. I thought the GPU was the issue then but I recalled that it was working fine in my own personal rig. There was also a beep code that had one longer beep and then three really quick short ones. I am honestly very discombobulated right now and debating what's the issue. Could it possibly be the Motherboard not recognizing the card?

[xdeathshot20]

prebuilts are generally really tedious about what you can put in, most likely it is just the board not wanting to recognize the card.  I dont personally know of any ways to fix that tho that wouldnt involve a whole motherboard replacement