When you tried today were you still in trial mode? Still 6km left?
Without posting any account details here do you pay direct through Zwift or use a third party payment option such as Apple?
Third party payment options often cause these types of problem but there has also been other reports of issues surrounding this 6km trial which might involve Zwift sorting at their end.
I’m sure others will be along with more help.
The bot should let you through eventually if you keep trying. You can also try support@zwift.com and hope for the best.