The error means that structuredClone was executed on Proxy instance, which cannot be cloned. In order to allow this, it should be used on raw object that a proxy wraps:
const b = structuredClone(toRaw(a.value));
Notice that toRaw is used on a.value because both a and a.value are reactive objects, and toRaw works shallowly and needs to be applied to the innermost object.
Since ref and reactive allow to compose reactive objects, toRaw still may not work for them due to how it works:
ref({ foo: { bar: barRef } })
This would require to recursively use toRaw on reactive objects before using structuredClone. At this point this doesn’t make it easier than cloning the objects manually, unless more exotic objects like Set, Map, etc are in use.