That GPU requires an 8 pin and 6 pin power connector. Do you have the proper connectors from the PSU? Also this forum thread may help.
I have recently upgraded my computers (Dell inspirion 5680) PSU and GPU, everything works fine with the PSU however the computer isn't detecting the Gigabyte 3070 GPU at all. It isn't appearing in the Device manager or speccy.   I've tested the old 1060 in the same slot and it works fine, I've disabled secure boot. I've made sure the gpu is properly seated. It's being powered by a 850w psu now so I'm sure it's getting enough power. I've uninstalled the older drivers but I can't seem to install new ones as the nvidia installer cannot detect the GPU so it just says they're is no device compatible. The fans do turn and the RGB on the GPU are lit up so it is receiving power. (The GPU is a Gigabyte Geforce RTX 3070 Eagle 8GB)
Dell INSP 5680, RTX 3070 upgrade not detecting