There is a possible chance that your WiFi is the reason that you are unable to set up your greetings for voicemail. Many users solved their problem by changing their network from WiFi to Cellular data. Your carrier provides the cellular data and that can be a better choice for voicemail settings.
If you want to add another condition to your rule, select Add condition. You can add multiple conditions to a rule but do this judiciously so your rules don't become overly complex.

To fix your voicemail not working on your iPhone, another method you can try is changing your voicemail password.
A good voicemail greeting is short and professional, lets people know that you’ll get back to them, and invites callers to continue engaging with a call-to-action. You should also show your personality if you’re in an industry or role that allows that. If your industry is more conservative, however, you’ll want to keep humor and personal touches to a minimum. A greeting Your name Your company A simple explanation for missing the call (e.g. you’re away from the phone or are on holiday) A rough estimate of when you’ll get back to the person An alternative person to reach out to (if you’re out of office) An alternative mode of communication (if you prefer email or text) A call-to-action such as “Leave a message” or “Send me an email at [email protected]”