Thanks for posting your issue on Ansys Learning Forum.
I would suggest you to refer, Help > Q3D Extractor PDF > Q3D Extractor Help
There is a section titled in Help PDF Assigning Terminals (Sources and Sinks), please refer this section which will be a good read for you.
You can assign source and sinks to any 2D or 3D object. Any net that you want to include in the inductance/resistance matrix solution must have a single sink defined, along with at least one source. This means that you can define multiple sinks to the same net but source should be only one. Please follow these guide lines with respect to source and sink assignment to overcome the errors.
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