What @richgalloway said, but whenever you reference a JSON field containing dots in the right hand side of an eval you MUST wrap the field name in single quotes, i.e. the first suggestion should be eval Error=case(isnotnull('attr.error'), 'attr.error',
isnotnull('attr.error.errmsg'), 'attr.error.errmsg') but for your solution the coalesce() option would make sense - note there the use of single quotes - always for the right hand side of the eval. This applies not just to JSON field names, but any field name that contains non simple characters or field names that start with numbers.
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