eSIM option missing on iPhone 12 Pro Max

Above is correct.

If your iPhone has two SIM slots, then it will not have eSIM, and eSIM support cannot be retrofit.

Find the model number of your iPhone, iPad, or iPod touch - Apple Support

If your iPhone 12 Pro Max model number is A2412 as I'd expect, then it is the dual-SIM model as was referenced above, and does not have and cannot retrofit eSIM.

Identify your iPhone model - Apple Support

If the phone was purchased in Macau, Hong Kong or China, it won't have an option for an eSIM.

What is the model numbers of your phone?