There are easy steps to build an inventory Manager.
Click on “Inventory”
On the right side click “Build Inventory Manager”
Select Any Channel I.e. eBay, Amazon, Shopify, Magento etc. to build “Inventory Manager”.
Note: “This uses the data from the imported listings of the selected sales channel to build your products in the sales channel. Choose the sales channel with the most accurate stocks level where you currently manage your inventory”
Click “Build Inventory” tab on the bottom right corner
Now you have made the selected Channel as your “Inventory Manager”