you need to upgrade the psu.  stock psu is 240w too weak for the gpu. how long is the video card?  If it is 336 mm it may be too long to fit the 3668 case.  The case is only 303 mm deep. motherboard has one 6+4 pin.  you can upgrade to SFX psu with 24 to 6 pin adapter.
Hello,   Can I install an NVIDIA GeForce RTX 3090 Ti 24GB GDDR6X PCI-Express 4.0 into a basic Inspiron 3668?   I already bought the GPU, and now I'm hoping to get information regarding any other required/optional upgrades I may have to do in order to get it to work, since it may be cheaper than having to buy a new PC. From what I understand, the power supply is a definite requirement for starters, but I don't even know which one to get.    Not good with PCs, and didn't think it through, I just thought that all my son needed to hook up his Oculus Quest 2 VR headset to his PC was a really good GPU.   Any help would be greatly appreciated!  
Basic Inspiron 3668 GPU upgrade to NVIDIA GeForce RTX 3090 Ti 24GB GDDR6X 
PCI-Express 4.0