Consider these two kinds of two-phase flow: (i) a gas and a liquid (the gas flowing at M > 0.3, and being the compressible phase) (ii) two immiscible liquids.

In normal applications liquids can be taken to be incompressible; hence both the above cases do not fit your description. If you want both phases to be compressible, you will need to consider two immiscible gases both flowing at M > 0.3. All gases are miscible (single phase) and I do not know of a mixture of two gases that are immiscible.

The problem you have chosen of multiphase flow, with more than one phase compressible, is unrealistic. Now you know why it shows error! 

Is there a huge pressure such a in petroleum reservoir? As Vijay as commented, Two inmiscibles compressible fluids is  strange scenaria. I can think  in a situation like that geological pressures such oil, water, and gas. You can put use some correlation to calculate the liquid volume change , or something like that. 

But if you is thinking in two gases - you have a single phase with two components, hence you have a multi component CFD simulation, not a two-phase fluid flow. 

Dear Sir,

Consider the following situation:

In a liquid rocket chamber, initially hydrogen (gas) and oxygen (liq) are injected and as time passes oxygen evaporates. while modeling the said situation in FLUENT, I need to use ideal gas formula for further calculation of density for both H2 & O2 gases but with 2 phase flow.

In this case, I need to model both hydrogen and oxygen gases in compressible form and fluent shows error.

Please help me with this.

Consider the atmosphere around us with N2 and O2. Is it single phase or two-phase?

As stated by me in my first answer and confirmed by Dr. Ricardo, your H2 and O2 mixture is not two-phase flow. Period!

Ok sir . Now i undersood and thanks for your answers.

As you have  first a liquid phase passing to gas phase, you must use a real gas thermodynamic model, ideal gas is not properly to this situation. The change from liquid to gas it will dependent on the thermodynamics coordinates at each point of your domain, and you be able to   simulate a sigle-phase mixture of O2 and H2 with a real gas model in all range of thermodynamic states. I think this was the source of your doubt. 

Vigneshwaran Sankar Can you help me with a similar problem, I'm working on a propellant tank pressurization problem in ANSYS Fluent. I'm unable to get the idea how to give phase conditions for LO2, GO2 and GHe in the system.