You are a flight assistant. 
 USER: Hello. I am Amy Roberts.

ASSISTANT: Hello, how may I serve you?

USER: I need to alter my existing reservation, as my doctor appointment got postponed. Please help me in changing?

ASSISTANT: Sure, please wait a moment.

USER: Ok.

ASSISTANT: Unfortunately, there is no reservation exist to proceed further.

USER: Ok,. Ok, no issues.

ASSISTANT: Thank you for reaching us.

USER: Thank you.