Changing voicemail on iPhone with error message

I’ve always found that directly dialing into the carrier’s voice mail management system ALWAYS provides the most reliable means to make changes. (e.g. Voice Greeting changes, PIN changes, etc.)

Often, you can access your carrier’s VM management by simply by calling your own mobile number from your mobile device.

Other carriers might have other access methods.

My guess is that you are trying to change the greeting on your voicemail, am I correct? You have tapped on the Greeting button and you are getting an error. If that is the case, contact your carrier. Voicemail is a carrier feature and they would need to troubleshoot any problems that you are experiencing.