30 January 2020
In recent versions (2019R1 and higher), this message often relates to an incorrect excitation (or at least perceived by the solver). User pabiri looks like their model excitation is set up correctly, so the problem is either due to an issue in R19.1 (an older version now), or due to the meshing being perceived as incorrect. Hopefully that explanation helps alexa410 also.
In addition to Mark's suggestions, I would add the following suggestions:
1) In general, I would suggest to make sure that the excitation cross-section matches the conductor cross-section exactly.
2) In general, I would suggest to make sure that if multiple current assignments are used on a conductor that they match correctly (pointing in the correct directions). And that the current either correctly goes in a loop, or it has an input/output definition that terminates on the boundary.
3) In the Initial Mesh Settings options, try adjusting the slider bar for Curved Surface Meshing further to the right (finer mesh, with larger total mesh count).
4) Try selecting all the surfaces of the conductor and assign an Insulating boundary.
5) You can always check that all 3D objects have the properties "Model" and "Solve Inside" checked.