How to make a product attribute appear in the Conditions dropdown of a Shipping Restriction/Shipping Rule?
Some attributes might not be available in the conditions list when you create a new rule. To add an attribute to the conditions, follow the steps below:
Step 1. Navigate to Stores > Attributes > Product, choose the attribute and open the Storefront Properties tab.
Step 2. Set the Use for Promo Rule Conditions setting to Yes.
Step 3. Click on the Save attribute button in the upper-right corner, and clear Magento cache.
After these steps, the attribute will appear as an option in the Conditions configuration.