I can’t find “personal hotspot” on my iPhone 12

Contact your cellular service provider and make sure they both support it and its provisioned for your service plan.

Link-> If Personal Hotspot is not working on your iPhone or iPad (Wi-Fi + Cellular) - Apple Support

your cellular provider is who controls whether or not you have the option to create a hotspot..

As Phil0124 has suggested, you should contact your cellular provider

from this article

How to set up a Personal Hotspot on your iPhone or iPad - Apple Support

If you don't see the option for Personal Hotspot, contact your carrier to make sure that you can use Personal Hotspot with your plan.

Did you by any chance change your cellular service plan in the year you did not use it?

As mentioned Personal Hotspot is controlled by your provider. If its not in Settings, then contact them and check that they still support it and its provisioned for your current service plan correctly.