That error means you're trying to store a value that's inside a repeat group in a hidden value outside the repeat group, or to save that value to a case, is that right? This doesn't work, since there are multiple values for that question, and CommCare doesn't know how to convert that to a single value. You might try using the join function to do this. Someone else had a recent issue that is similar, check that thread for more details:
There are also some other old posts describing the issue that might have more information: Search results for '"more than one node"' - CommCare Forum