There’s another thread about it already:
And I’ve been in communication via email with someone at Woot about it (they emailed me yesterday) so I know it’s being worked on.
I can say if you have any ad blockers enabled on your phone, try disabling them and then try to checkout again. That’s what fixed it on my phone as well as at least a couple of others. That doesn’t seem to be the problem for everyone, though.
(Please note: I don’t work for Woot, I just volunteer to help out here on the forums.)