In case it's missing from Item's Extended Properties then it may be taken from Linnworks.net > Shipping > Integrations > Services > Edit screen from the Default HS Tariff Code field. This field is not present for all courier integrations, but some may have it per courier shipping service.
In case it's missing from the service configuration, then it will be checked from Linnworks.net > Shipping > Integrations > Config from the same Default HS Tariff Code field
Please make sure to have the tariff code present in your Item's Extended Properties in case you are shipping different product types or you can add a Default HS Tariff Code for some couriers in case you are shipping only one type of products. You can lookup the correct tariff codes from the following website:
Additional validation for the length of the tariff code was added due to ICS2 regulations that came into effect on the 1st of March: