For the case of semi-permeable interfaces, the output program shows the resulting specific groundwater flow through the interface elements (qn for 2D and 3D).
These output results and their plots are not available in the case of fully impermeable or fully permeable interface elements.
Specifically, in the case of fully permeable interface elements, the pore pressure degrees of freedom are fully coupled in the calculation kernel. Since the nodes of the interfaces are fully coupled there is no difference in head and as a result, Darcy's law is not applied and the specific groundwater flow cannot be calculated.
A workaround for this specific case is to specify a very low value for the hydraulic resistance (d/k) for the interface cross permeability.
Permeability in interfaces
[Tips and Tricks]
Permeability in interfaces: Practical situations
Using permeability in interfaces
Modelling soil-structure interaction: interfaces