The offset tool says "you must select a single face or two or more simply connected coplanar edges."
Actually the selected edges must also be not colinear.
Also, a single edge should be allowed if it part of a single face or part of only two coplanar faces.
Multple faces and/or multiple unconnected edges should be allowed if they are all coplanar.
Finally ARCs should ALWAYS result in offset ARCs not CURVES, This is a nasty limitation because CURVES do not have centers and can’t be dimensioned as ARCs.