First, let's make sure you are doing this the right way. Here's how you change the email address you use for your Apple ID while retaining everything in your account:
1. Go to https://appleid.apple.com and sign in.
2. In the Sign-In and Security section, choose Apple ID.
3. Enter the email address that you want to use as your Apple ID.
4. Choose Change Apple ID.
5. If you changed your Apple ID to a third-party email address, check your email for a verification code, then enter the code. Learn what to do if you didn't get the email. --> If you didn't receive your verification or reset email - Apple Support
6. If you use services like iCloud or Messages to share with friends and colleagues, sign in to those services with your updated Apple ID. --> What to do after you change your Apple ID or password - Apple Support
For more details, read the document: "Change your Apple ID" --> Change your Apple ID - Apple Support
If the above is not working, where exactly does it fail? Are you sure you have the correct address? Apple should be sending you a verification email to that address. Apple does not say what happens if it cannot send it but maybe a "fail" message means that there is a problem with the address you have entered.