Are you getting this error when attempting to sync invoices from WHMCS to QuickBooks Online?
Export Product Error – Invalid product account.
Please choose an income account in Module Configuration -> Default QBO Product Account.
Visit Settings > Module Configuration in our module menu bar (also accessed by Setup > Addon Modules > WHMCS QuickBooks Sync in the WHMCS admin menu) – and make sure you have a valid Sales Account selected for the Default QBO Product Account setting. Then, save the module configuration and try again.