esim option not showing on my phone

There are two reasons for this:

1) Your carrier does not support eSIM. Contact your carrier to ask them about this.

2) Your phone was originally sold in China (where eSIM was not supported).

Tap Settings > General > About > Model Number

Tap on the Model Number and another 5 character number will appear. Example.....A2408

What is the number that you see?

Per the above reply (Settings > General > About > Model Number > tap on the model number), the iPhone 12 Pro model A2408 is a dual-SIM model, has no eSIM support, and eSIM hardware cannot be retrofit.

A third possibility is a hardware problem with the iPhone eSIM hardware, if this is not iPhone 12 Pro A2408, or some other dual-SIM iPhone model.