Same issue i have been seeing all over the internet. I cant get past the "We're getting your payment ready" screen. Trying to get back into the game and pay for a membership. Cant even make it to the payment selection screen. I've tried multiple different browsers, i've tried incognito mode, ive tried my cell phone. nothing is working at this point. How am i supposed to pay for membership?
I recommend waiting until tomorrow, then trying again.
Often those error messages mean there is a problem with Jagex' payment processing system and all you can do is wait for them to fix it. They WILL notice soon enough
Purchases directly from Jagex via credit card should process immediately.
The rejection is not coming from your end (bank/credit card provider) as you would see a different error message.
p.s. I see you have membership now so, maybe you have resolved the problem already.