Your post is a bit unclear.
Are you having problems with updating your payment card details?
Or with actually trying to purchase/pay for a membership plan?
What exactly are you trying to do, what steps do you take to do it, what goes wrong, any error messages etc.?
If you are using the app then try the web interface instead in case it makes any difference - or vice versa.
The links above are for the web interface (I tend to avoid the app as I often find it flakey).
When were you last able to successfully buy a membership plan?
If you need to contact 48 support for further assistance then see the links in my signature below.
