The interface that cannot be obtained is 

1. Liquid-liquid

2. Solid-liquid

3. Liquid-gas

4. Gas-gas

 
 
 
 
HINT:  Gases are miscible with each other.
 
Explanation: 
 
For interface formation , one substance should be in immobile or steady state. Gas-gas interface can not be obtained as they are completely miscible in nature. e.g., air is a mixture of various gases such as O2, N2, C2, etc.