Nagarajan D
It seems your model has atleast one surface body. Please note that selection of such shell body as physics region is only allowed with pure structural physics. Since, you are working a fluid physics, any attempt to assign shell body as fluid region will lead to this error.
Please review the 9 volumes that you have selected as fluid region and remove any surface body that might be present. This should do away the problem. I hope it helps.
Regards,
Gaurav
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