Hi again Furkan!
We already discussed about this briefly in our chat, but we can of course expand our discussion here
There are a few best practices when creating an open inner region. One of them is that all parts should be merged together, especially in this kind of simulation, where the Keep existing parts option is not supposed to be activated during the flow volume operation.
In case you wanted to keep the parts separately, let’s say if this was intended for a conjugate heat transfer analysis for example, the solid parts that extend to the outer part of the domain would still make the extraction to fail. It is important that the program “understands” which is the inner and which is the outer region, so inconsistencies like open gaps/overlapping parts etc should be avoided.
I would advice you to merge everything into a single solid part as a start.
Let me know what you think, and everyone is welcome to add feedback of course!