Hi Maxim, I’m one of the engineers at Pulumi. Sorry for the 500 error 😓 .
The root problem is that your Pulumi subscription doesn’t have a payment method on file. So when we try to change the subscription’s billing interval, our payment processor is rejecting the request. If you first attached a payment method (e.g. adding a credit card on your organization’s billing page), then the request to change it from monthly to annual billing should go through.
If you have any other questions, or things still don’t quite look right please let me know. Cheers!