Many people will create their own SKU's and ASIN's on Seller Central. When this happens in many cases those ASIN's/SKU's have not been pulled in by Seller Champ for use.
In cases like this manually refreshing your Amazon account is the fastest way to resolve this. It will pull those ASIN's and SKU's in for you to use in a manifest. Go into Settings -> Marketplaces ->Amazon and click the box with 3 hashmarks in it; then select 'Refresh as shown in the screenshot below:
In about 15 minutes your account should be refreshed and the item should show up in your inventory for use.