If the RX 6500 requires a PCIe Power cable to be connected to the GPU card you need to connect it otherwise it won't operate correctly but most likely it won't turn on without the GPU power cable connected.
You can purchase a Molex-PCIe Power cable to connect the GPU card or some other type of adapter.
As an example only from Amazon:
Your GPU card will run fine using PCIe 3.0 x 16 slot.