Thanks for your posting. Test / Requirement Description seems to be a custom field in your TestRail installation and is marked as required (under Administration > Customizations). Required custom fields must also be submitted to add_case/update_case and not doing so results in the error/validation message you’ve received from TestRail.
You can submit custom fields via the API with their system name prefixed with custom_. For example, if your custom field has a system name of requirement_description, you would submit it as custom_requirement_description. The system name for custom fields can also be looked up under Administration > Customizations.
I hope this helps. Just let me know in case you have any further questions.