Dell octiplex 5050

I cannot tell which model of GPU you are trying to install. But be that as it may this statement from Hardware-Corner indicates there is not a 6 or 8 pin adapter on the 5050 PSU:

 Typically, the stock power supply in a business PC does not have the necessary 6 or 8 pin connectors to power the GPU. In such cases, you may need to use a low-power GPU or a SATA to PCIe power adapter. Alternatively, you can change the entire PSU.

You can find that adapter at Amazon here.

if your stock psu has no 6 pin PCIe gpu connector you can upgrade the psu to Dell oem 360w which fits the 5050 like a glove and has the needed 6 pin gpu connector.

GPU shown needs 8-pin connector and will trip or fry current OEM PSU.

Some compatible GPU's that go with current PSU:

• 30w GT 1030 GDDR5
• 30w Quadro P400
• 50w RX 6400

A couple more are in the spec sheet screengrab:

And a couple more ideas.  Some even newer Optiplex's have optional higher watt PSU's.  The XFX card looks more suited for that or a Precision.  In the other direction are somewhat older Optiplex's that were full-size MT's and could take full-size aftermarket ATX PSU.  But I doubt that's the direction you would want to go in.

If you want to revolve around the GPU card vs. the PC, what model card is it?  Then we can give some tips on getting it powered with the correct PSU and/or mini tower.