Problem:
A. SKU Without Inventory
Navigate to Masterfile > SKU Listing.
Search the for SKU. Click on the SKU and click "Edit SKU".
Check whether the "SKU Without Inventory" is set to "Yes". If "Yes", change the "SKU Without Inventory" to set as "No".
Scroll to the bottom and click on Update when complete.
B. SKU Without Inventory Inherit from Category
If the "SKU Without Inventory" is set to "Inherit", like in the screenshot below.
Navigate to Masterfile > Category > Category Listing.
Click on the Edit icon to check the category and subcategories in question, and see whether the "SKU Without Inventory" is set to "Yes".
If found "Yes", change the "SKU Without Inventory" to set as "No". Click on Save when complete. (Only change this option if the Entire Category / Subcategory is supposed to have Inventory. If only for one specific SKU, change to "No" in Edit SKU.)
