On the Item card, you can see that serial number tracking can be switched on/off at the item level (per product).
Note: Serial number tracking can only be enabled for finished products
Since serial number tracking can be set at the item level, this setting will also affect all variants of the item. Enabling/disabling serial number tracking for an item will only affect that item.
Manually enabling product serial number tracking
Navigate to the Items screen and select a product from the list.
Enable Serial numbers.
NOTE: Serial number tracking in Katana is available in the Advanced, Professional, and Enterprise plans (see more about subscription packages). If you're on the Essential plan, you will be asked to confirm the change to an Advanced (or higher) plan. You can choose a monthly or annual plan with a free 14-day trial.
Serial tracking for all variants of the item are now enabled.