On an older project I was prompted to change to the new environments:
However when I press Yes, I get this error:
Invalid environment URL - this URL is not available
I get the same error when I want to delete the Default environment.
Any ideas what is going on here?
PS The reason in the first place to want to change the environment was because an HTTP api this app provided ceased to work.
I’m having the same problem, when I click on “switch for public link” and copy my custom domain, bought from Go Daddy, I get the following error:
“invalid environment URL - this URL is not avariable”.
What could be the reason?, since the URL exists and is not being used.
Welcome to the forum! 
In future please try to ask your question in only one place. Otherwise, someone may take time to answer a question that has already been answered elsewhere, which can be frustrating.
For the reference of others, this has been addressed elsewhere: