Your screenshot already uses TO_VARIANT_OBJECT; the VOID field inside the STRUCT is what causes the failure.
TO_VARIANT_OBJECT requires every field to be convertible to VARIANT, but VOID is not accepted in that nested path. Scalar CAST(PAYMENT_DATE AS VARIANT) works because untyped NULL can be cast directly to VARIANT.
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If the intended type is known, cast it at the source, for example CAST(NULL AS DATE), then use NAMED_STRUCT to preserve the field name.
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If the type is unknown, use TO_VARIANT_OBJECT(STRUCT(CAST(PAYMENT_DATE AS VARIANT) AS PAYMENT_DATE, TRANSACTION_DATE)).
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For unknown or wide schemas, PARSE_JSON(TO_JSON(STRUCT(*), map('ignoreNullFields', 'false'))) handles VOID fields, but converts DATE and TIMESTAMP values to strings.
