To know the procedure for creating the master sku click on below link and follow the steps mentioned in pdf.
https://support.easyecom.io/portal/kb/articles/steps-to-create-master-sku-in-easyecom
Step 2: Go to Primary account --------> Inventory ---------> Manage Inventory, Drag the purple Burger and click on Map sku across the accounts
Step 3: Select the child account and click on download unmapped sku list
Step 4: Fill the Parent Sku column and then save it
Step 6: If you want to check the mapped listing, select the account and click on Download Mapped.