Hi @OscarV2, thanks for posting
Since I don’t have a link to your project in particular, I’m not sure the exact context behind the error. However, I would assume you have two solids touching each other (for instance, a solid part and the flow region) and are trying to have the wall boundary condition assigned to the interface. As the error message states, however, this is indeed not possible in SimScale.
A workaround I’ve seen working in the past is to create a small cavity inside the solid and apply the temperature there. Since the thickness will be too small for a gradient, the outer face should conserve the temperature.
Another alternative is to go for a power source instead - in this case no CAD changes are necessary.
Cheers
Igor