Two potential things that could prevent it working.
- Your case may only have Low Profile slots. So the width of the case isn't enough to allow the card to go in.
- And the Power Supply inside may not (probably wont) have the right connectors or wattage to sustain that card.
Easy way to find out is to bust open the case and check.
If the graphics card already in there (a GT520) is of Low Profile size, then you can only put in low profile cards, which will limit what cards you can get.
Next you want to look for the electrical specifications on the PSU, given its a pre-built it would be at the top of the case. Need to find the amperage (A) that the 12v rail/s are running at. That will tell me the wattage of the system. Will look something like this
Also look through the cables coming out of the PSU. What you need for that card are two 6-pin connectors that look like this. If they arent there then you will likely need a new PSU.