setting up new iPhone 12 from 6s

I checked and the iPhone 6s uses the same sized nano SIM card. I assume you got the sim free version versus a carrier option. If fir some reason your nano SIM card isn’t fitting you would need fo have your carrier provide you with a new one. That may be a good idea anyway so you can get the 5G SIM card. Someone on here has repeatedly said there is no such thing but personally I had to switch to the 5G SIM card that I got with my phone because using my previous one did not automatically provide me access to the 5G service so regardless it does not hurt to get it. Only thing I can think of is your SIM card is damaged or it’s not being installed correctly (seems simple but it happens). Unless your old iPhone is older and used the micro sim (which was before the 6/6s series....maybe back to the 4 or before).

iPhone 6s and iPhone 12 both use Nano-Sim. There is no difference in the shape of Nano-Sim.

That said, I would recommend you get a new sim card from your cellular provider for iPhone 12 as you'll want the latest for 5G cellular service.

Can’t recall if the 6s uses the nano sim (pretty sure it does) but the sim should fit. Did the new phone come with a SIM card? If so and it’s for the carrier that you use then you can use that.