According to the blog post since september/2020 and up to december/2020 instance type t4g will be available. I'm trying to create a machine, on one of the right zones (Oregon) according the FAQ, and using the right AMI, also according to the FAQ
So why I cannot choose those machines?
As you can see on the screenshot, t3a are eligible, but all t4g not:
Solution was pretty simple: you have to choose ARM architecture on the first step, AMI selection. Accustomed to choose x86 architecture, I didn't pay attention to that radio button. Also I didn't bother to hover over the forbidden icon on the first column in the image posted on question, which clearly stated what was the problem:
So solution is to select ARM on the previous step:
