The MP3 encoder requires additional activation explained on this page. Usually that is done over an internet connection. That page described activation on another computer or over the phone, but I doubt those work in Vegas.
I'm having the same problem in Vegas Pro 14. Cannot render an mp3 and get this error prompt;
I've got Version 14.0 (Build 244). Product has been registered and my serial number is visible in the "About" section. Never seen this prompt before. I added Vegas on my Windows (10 Pro) firewall allow list to see if that would enable it to connect, or do whatever it's trying to do, but no dice.
I went to the "additional activations" section of the link provided above, but there are no instructions on how to perform these in Vegas. Guy in the video tutorial is using "PhotoStory on DVD" and his 'export' menu has different options than what I see.
If anyone knows how to remedy I'd appreciate it very much. This is so annoying and unnecessary. I miss the Sony days.