[immortality]

what is Ydefault file that solver complaint about?

Create mesh for time = 0

Reading thermophysical properties

Selecting thermodynamics package hePsiThermo<homogeneousMixture<const<hConst<perfectGas<specie>>,sensibleEnthalpy>>>
--> FOAM Warning : 
    From function polyBoundaryMesh::groupPatchIDs() const
    in file meshes/polyMesh/polyBoundaryMesh/polyBoundaryMesh.C at line 474
    Patch empty specifies a group empty which is also a patch name. This might give problems later on.


--> FOAM FATAL IO ERROR: 
cannot find file

file: /home/ehsan/Desktop/takhlie_4_p_const/0/Ydefault at line 0.

    From function regIOobject::readStream()
    in file db/regIOobject/regIOobjectRead.C at line 73.

FOAM exiting

[adhiraj]

When you have multiple species, you can specify their initial mass fractions. So you would have files named O2, CO2, and so on. Any species that is not specified explicitly is read from the Ydefault file.