I think revolut does not support 3rd factor authentication so you can't use at those online website that required them, same issue as youtrip basically.
How come the Revolut site say it does? Or not yet for SG?
https://blog.revolut.com/3ds-secure-payments/
next time try to understand how the exchange rates works or at least read a few pages of the thread first before you complain. at least won't make you look stupiak..