I want to update my field "Confidential Level" to new value "Class-A."

Setting 1:

Custom field ID=Confidential Level

Field value=Class-A

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message error:

WARNING: jira rest client update fields error for issue SUPPORT-113
java.util.concurrent.ExecutionException: RestClientException{statusCode=Optional.of(400), errorCollections=[ErrorCollection{status=400, errors={customfield_Confidential Level=Field 'customfield_Confidential Level' cannot be set. It is not on the appropriate screen, or unknown.}, errorMessages=[]}]}

what means "It is not on the appropriate screen"???

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Setting 2:

Custom field ID=10406    (I input the custom field ID to here)

Field value=Class-A

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message error:

WARNING: jira rest client update fields error for issue SUPPORT-113
java.util.concurrent.ExecutionException: RestClientException{statusCode=Optional.of(400), errorCollections=[ErrorCollection{status=400, errors={customfield_10406=Could not find valid 'id' or 'value' in the Parent Option object.}, errorMessages=[]}]}

It seem it cannot find id or value.

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