What Thinkpad model is this? Would be easier to look up supported hardware with that information. I think it's missing a drive cage, I don't see any mounting hardware in that 2.5" area. And likely will need a ribbon adapter, if the model supports one.
There is also a possibility this is just empty space in the chassis unfortunately.
Edit-: JHDD2 should work with a ribbon cable adapter, you'll still want a drive caddy to keep it secured.
Thank you for your help. I found the cable. I didn't think of a tray but will be sure to order one.
Thank you
Search the model for a repair guide. That should tell you all you need to know.
I was looking up how to connect. I see there is a cable that is required. I have ordered it.
For refrence if anyone else has an issue like me. https://www.ebay.com/itm/296024619796?chn=ps&amdata=enc%3A1KqAehV8GTyKSqT1brSfK-g10&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&mkscid=101&itemid=296024619796&targetid=2299003535955&device=m&mktype=pla&googleloc=9027222&poi=&campaignid=21214315381&mkgroupid=161363866036&rlsatarget=pla-2299003535955&abcId=9407526&merchantid=767752053&gad_source=1
Easier with the model # of the Thinkpad so people can look up the specs, but working off what you posted and the pics..
and one 2.5" SSD/HDD slot
Have you tried plugging in the 2.5" SSD there & then boot into the BIOS to see what it detects? (your 3rd pic, the slot on the right looks like it could take a 2.5" SATA drive) Carefully... just as a test run - Like the other comment said there isn't a drive cage or anything there so you couldn't really run it with a 2.5" drive full-time until you can rig up something to mount it properly.
This particular Thinkpad may have only been set up for M2 drives so getting a 2.5" drive working there could be tricky, I'd just get a compatible M2 drive to install in there since you know for sure that's what the Thinkpad was configured to work with previously.