The constraint not(selected(., 'dont_know')) or count-selected(.) = 1 should work for you. This says that either "don't know" isn't selected or if it is only one item can be selected.
Hi
I have given not selected name is "98"
and I wrote this code in constraint
not(selected(., ‘98’)) or count-selected(.) = 1
and its shows this error
Error: Problem with JavaRosa Parsing Form: org.opendatakit.aggregate.exception.ODKIncompleteSubmissionData: Javarosa failed to construct a FormDef. Is this an XForm definition?