The motherboard is a mATX type and yes as
@Nikon f1.8 Paul says that is indeed the slot that can be used as needed.
However, it is the only such slot and raises a question of .....is your monitor connected to the motherboard DP port.....or is perhaps connected to a separate graphics card (GPU) that is already in the single PCI slot?
Also, I note as in "3" on the motherboard annotations here
https://rog.asus.com/uk/motherboards/rog-strix/rog-strix-b550-i-gaming-model/
That there are 2 off M.2 slots on the motherboard, are they both occupied as you mentioned that you don't have a spare available? NB I note on that page that the two M.2 slots are not equal, one complies with PCIe version 4.0 and the other is version 3.0 compliant.
On the surmise that you have both, the empty PCI slot and actually do have an empty M.2 slot (and whoever built your PC used the faster v4.0 position) on paper it seems you are best to take up Paul's kind offer and use that for the best speed.
Edit
I think the board is iTX not mATX
From the manual here
https://rog.asus.com/motherboards/rog-strix/rog-strix-b550-i-gaming-model/helpdesk_manual/
It seems the design is IMO unusual in that the second M.2 slot is on the rear/bottom side of the motherboard. So, either to install it you would need to remove the motherboard or just possibly maybe if the case design has cutouts in the frame to access the reverse of the motherboard without complete disassembly to make the fitting?