Convective Heat Transfer Internal Flow Volume

I am trying to create an internal flow volume but it does not allow me to. I have checked with the support team and the gap tool, it detected no gaps. I am wondering what is the issue?

Hello @h_tai0220201 , and thanks for reaching out to us via Forum!

If possible, could you please share a URL to your project here so that we can have a look at your model?

Thanks, Kaan