I had the same problem and the following response from Zoom support was a successful workaround for me:
To start, we can try and troubleshoot your issue by updating your billing information and saving the card on file before you process the upgrade.
To update your Billing Information, please follow the steps below:
1. Sign in to the Zoom web portal.
2. Click Account Management and select Billing.
3. Click the Billing Information tab.
4. Click Edit next to the contacts you need to update.
5. Update the contact information.
6. Click Save Changes.
For reference, please see: How Do I Update My Billing Contact? – Zoom Help Center.
After updating your Billing/Sold To contact, you can then update your Payment Method:
1. Click Edit next to Payment Method.
2. Select between Credit Card or PayPal.
3. Make sure to uncheck the "Same as Bill To contact" checkbox then enter the card details and billing address registered with the card by the bank.
4. Click Save Changes.
For your reference, please check: Update Credit Card - Zoom Help Center.
Once you updated your Billing Information, you can process the purchase here: zoom.us/billing by clicking "Upgrade Account" or here: https://zoom.us/pricing. For reference, please see this page: Subscribe to a Paid plan – Zoom Help Center.
Should you encounter a payment error upgrading to Pro, you can check this page on Error codes for billing – Zoom Help Center for more information and troubleshooting. You can also call us at 1-888-799-9666 and press 1 for Sales to assist you in purchasing a subscription.