When attempting to define a "thermal coupling - convection" and the user cannot select a curve or curves for a fluid duct. The curve will select outside the thermal coupling form, but not when defining the fluid ducts simulation object.
Solution
To be able to select a curve as a fluid duct it must be meshed with a 1D mesh defined as duct elements. If there are no valid elements on the curve, then the form will not allow the user to select this curve. Double check that the curve is meshed and that the 1D elements are defined as duct elements.
Notes