Unable to change email address due to locked Apple Account

For locked account look here:

If your Apple Account is locked, not active, or disabled - Apple Support

To sign out of an Apple Account on a device, it can only be done in one of two ways:

• Provide the password to the account. With a locked account it may be possible to gain access by resetting the Password.
• Provide the original purchase receipt from Apple or Authorized Apple Seller to remove the account.

https://al-support.apple.com/#/additional-support

If you have more information about how your account became locked or possibly if this was a recently purchased used device, it may be beneficial for specific information.