Sorry for the issue there.
Would you be able to confirm your PayPal account status, and if confirmed try to de-sync and re-sync the PayPal / RevStream option on your GoToTraining account?
This happened to me as well - got a message that the payment transaction could not be processed. Called GoToTraining support, who said that it was a PayPal issue and nothing they could do on their end. Suggested I check payment settings on the Admin panel.
There were no issues or recent changes that affected the Admin settings.
About an hour later, payments were able to go through. So appeared to be a temporary problem. Incident occured in late evening.