See explanation below.
Explanation
You need to create a unit of measure for the pallet and another unit of measure for the box (if they don't already exist). Then you create a unit conversion.
Create a unit of measure for pallet and box:
* Go to Navigation pane > Modules > Product information management > Released product maintenance.
* Click Units.
* Click New.
* In the Unit field, type a value. Enter the ID or symbol to use when referring to the unit of measure.
* In the Description field, type a value such as Pallet. Enter a descriptive name for the unit of measure in the system language.
* In the Unit class field, select Quantity
* In the Decimal precision field, enter a number. Specify the number of decimals that the converted unit of measure must be rounded to when a calculation is completed for the unit of measure.
* Click Save.
* Repeat the above steps for Box.
Define unit conversion rules
* On the Action Pane, click Unit conversions. Define rules for converting the unit of measure to and from other units of measure in the selected unit class.
* Click New to open the drop dialog.
* In the Factor field, enter 12. Conversion factor between the From unit (Pallet) and the To unit (Box).
* In the To unit field, select Box.
* In the Rounding field, select To nearest.
* Click OK.
* Close the page.
Reference:
https://docs.microsoft.com/en-us/dynamics365/supply-chain/pim/tasks/manage-unit-measure