What should I do if I can't renew my Non-O visa before leaving Thailand?

Yes

The amount of stupid people who post idiotic shit in this group, Thailand needs to seal its borders and keep you out

How long have you gone—can renew 3 months before—why did you not do before you left

Have you checked how early you can apply for the extension?

If you come in on a tourist visa you will have to go through the process of applying for a 90 day Non-O visa inside Thailand, which will involve showing you have a minimum of 800k baht in a bank account, possibly transferred from overseas depending on the immigration office you use. When that 90 day visa is down to it's last 30 days you would apply for a 12 month extension. You will likely find it much easier just to apply for the 90 day Non-O visa before you travel rather than coming in on a tourist entry