It's not possible to create a new version for any App Store Connect app.
To reproduce, I navigate to my app and click the (+) button to create a new version number. I type in a version number, and click "Create."
It fails with an error, "An error has occurred. Try again later."
When I view it in the Network tab of the Web Inspector, it shows that there's a 500 error.
{
"errors": [{
"status": "500",
"code": "UNEXPECTED_ERROR",
"title": "An unexpected error occurred.",
"detail": "An unexpected error occurred on the server side. If this issue continues, contact us at https://developer.apple.com/contact/."
}]
}
The same error occurs when I attempt to create an app store version using the App Store Connect API.