3020 motherboard uses Dell proprietary 8 pin ATX so Be Quite! system power 9 (600w) w standard 24 pin ATX would not work without adapter. You can try a 24 to 8 pin adapter 

https://www.amazon.com/Eyeboot-8-Pin-Supply-Adapter-Motherboards/dp/B01A210HKO

https://www.ebay.com/itm/12-Inch-24-Pin-to-8-Pin-18AWG-ATX-Power-Supply-Adapter-Cable-for-Dell-Computers/222545479269?epid=675569317&hash=item33d0be8665:g:GKcAAOSwYXVY0MB7

(cheaper if you buy from overseas)

RX580 is a power hungry card. move to tower case w good air flow is helpful. It will fit the PCIex16 slot on 3020.  Alternative nvidia card is GTX 1060 (6gb), 400W psu required.

when moving to non-Dell case you need to rewire Dell proprietary front panel connector which can be a small challenge. You need to search for pinout.

Thank you very much for your response, i understand that it will be hard to transfer everything into a new case so can you please 

1. Recommend the best GPU that will work with the default power supply

2. Recommend any 400w power supply 8-pin ATX connector (if there is any like that)

A low profile GTX 1050 or 1050 Ti will probably work w stock 255W psu if you do not push for peak gpu performance. A low profile GT1030 will certainly work but imo the performance cost ratio is not great for this entry level card.

An easy upgrade to Optiplex XE2 slim psu 315W (DP/N VX372) will definitely fit the case and support 1050/1050Ti.  https://m.youtube.com/watch?v=I7_enEWxJyM

dimensions: 9.5 x 2.5 x 3 inches

A possible 400W psu is from HP pavilion slimline. not certain it will fit Dell SFF case. Dimensions: ( W x H x D ) 3.4" x 2.5" x 7"
Connector included: ATX Connector 24 (20+4) Pin X 1 PC, P4-12V 4 Pin X 1 PC, SATA 15 Pin X 3PC, PCI-Express 6 Pin x 1 pc

Be careful with which GPU card you get. I bought an ASUS GTX1050 and the heatsink was way too big. The fins on the heatsink hit the cover of the PSU, which extends across the entire bottom of the case. Plus, Dell in its infinite wisdom ran a pair cables along  the PSU which not only interfere with the card, their too short to move out of the way.

I have been looking at a Nvidia T400 4GB GDDR6 for my 3020 SFF do yall think it will work?

https://www.amazon.com/gp/product/B09TBLVX9K/ref=ox_sc_act_title_1?smid=A2KN9K54F1HXTQ&psc=1