It is only open for certain hours -
On Saturday, Jan. 6., the form is expected to be live from 12:30 p.m. to 6 p.m. ET and 11:30 a.m. to 5 p.m. ET on Sunday.
S24 and I were able to complete it during one of their open windows with no problems. However I think the IRS link is down, so this weekend might not be the best time to fill out the form.
During the week they are expected to be open between 8am-8pm EST. Hours will be posted on the website. Good luck.
Edit to add: Right now it is supposed to be open, but I just checked and it is not available. So I guess those hours are not really very useful.