Perhaps, seeing the project will be helpful. Here is a previous post from the forum on how to share a project:
Also, perhaps the internal flow volume was not created successfully. When selecting the seed face, select any internal face that will be in contact with the flow region.
When selecting the boundary face, ensure that there is a distinction or a clear boundary between the internal and external domains. In other words, to select the boundary face select the faces that lie between the external environment and the internal.
Follow along this tutorial for more information.