When you want to find out what Accounts do not have Contacts associated with them, create two lists using ListBuilder.
First, create a Contact List. Click HERE to learn how. Once the list is created, export to Excel.
Second create an Account List. Click HERE to learn how. Once the list is created, export to Excel.
You need these two lists so you can compare them, since the Contact list will only have Contacts that are already associated to Accounts.
By comparing the two lists, whatever Accounts that are on the Account List but are not on the Contact List, are the Accounts that do not have Contacts associated with them.