Are you absolutely sure your PC meets all other requirements for MR? If your PC is old enough to have PCI and not PCIe (which leads me to believe it's 6+ years old) then you absolutely need to check that everything else is supported first.
Does the Mixed Reality Portal app only show issues with the USB?
This card might work, it would probably let you continue setup, but it might not be optimal (the card doesn't have the full speed of USB 3.0).
PCI is only 1/5th the speed of USB 3.0 - it is much, much slower. You will likely run into issues.
If you have no USB 3.0 AND no PCIe at all, you will almost certainly need a new motherboard and CPU. If you are sure that you have neither, your system, simply won't be fast enough.
Now, if you have a single PCIe slot for a graphics card, you might be able to make it work. MR requires a relatively recent graphics card with DirectX 12 - If your card is more than 5 years old (or has a PCI connector), it's not gonna work. But otherwise, it should be fine.
However, the USB 3.0 is not something you can do much about - if your system literally does not have a connector that is fast enough. You can absolutely try that card if your system meets all other requirements, but it may not work very well, if it works at all.