in what way does my question entitled you to be rude? I'm asking what the reason is. You know, there may be one. 🤷 Ideally it's sensible to obtain non O before arriving. Open bank account. Then get 12 month extension.
"they won't let you open one"? Not true. The people who have difficulty in opening bank accounts are usually the ones who don't obtain the relevant visa before arriving. They turn up as tourists and then try and open an account.
Are you in, or travelling from, one of the countries that are flagged for yellow fever? If not, then you don't put anything because it's irrelevant to you. Make sure you're on the Thai official page for the free arrival card.