Your product has a unique identity, assigned by the Store.
If you are building your package manually (that includes MSIX tool) , you'll need to include its identity details.
This is the way I do, probably there are other methods as well:
Go to your Microsoft Partner Center - Dashboard,
Select your Application,
Go to Product Management,
Select : Product Identity
You should be able to see on the right hand side pane the Values that will need to be included in the Application manifest (i.e. your MSIX Package) for the:
Package/Identity/Name
Package/Identity/Publisher
Package/Properties/PublisherDisplayName
Copy those and use them in your MSIX Package Editor (i.e. in the Package Information section).
Also, make sure you select an operational Time Stamp server for validating your (pfx) certificate.