Answer
This is almost always a template mapping mistake elsewhere in the job rather than a problem with the TAX_LIABILITY_TYPE value itself — check whether another field's mapping (or a default value meant for a different column) is accidentally pointing at TAX_LIABILITY_TYPE's position, or whether TAX_LIABILITY_TYPE's own mapping is pointing at the wrong source column, so the value the framework actually receives for TAX_LIABILITY_TYPE ends up null despite the source file/default looking correct at a glance. Carefully re-checking every field's mapping in the template around the affected column (not just the column that's throwing the error) is what typically surfaces the actual misconfigured mapping.