resolver-not-registered

[wizzard] no resolver is registered as "<name>". The flow names it in a $ref, and the registry it
is evaluated against has no entry by that name. Add <name> to the registry, or correct the name in
the flow. …/errors/resolver-not-registered

Thrown as a WizardError from three places, and op says which:

op	When	path
evaluate	a synchronous expression - a when, a transition guard - is read	not set
evaluateAsync	an expression is awaited	not set
validate	a step’s validate is run, by next() or by validate()	steps.<id>
load	a step’s load is run as the wizard enters it	steps.<id>

A flow is data, so the functions it needs are referred to by name and supplied separately. The name in { "$ref": "checkEmail" } has to match a key of the registry passed to createWizard exactly, including case. The usual causes are a typo on one side, a resolver that was renamed in code and not in a flow that came from a server, or a registry assembled conditionally that left this entry out.

const wizard = createWizard({
  flow,
  registry: {
    checkEmail: async (_args, { data }) => (data.email ? null : { email: 'Required' }),
  },
});

Refusing is deliberate. Treating an unknown name as false would make a when silently hide a step and a validate silently pass, which is the failure that is hardest to see. To catch the mismatch before the wizard runs, pass the registry to validateFlow or assertFlow: both report a $ref the registry does not hold, with its path, as a problem with the code resolver-not-registered. A $ref in a step’s ui, in another $ref’s args or beside a flow’s steps is your data - a JSON Schema’s, say - and is never looked up, so it is not reported.