FAQ - ExpensiError QBO056: Expenses are not categorized with a QuickBooks Online account
One or more of the categories on the report listed are not showing as synced from QuickBooks Online. Go to Settings > Policies > [Policy Name] > Connections and sync your QuickBooks Online connection. Once you have successfully re-synced the policy to QuickBooks Online, re-apply the categories on your expenses and try exporting again.
If that doesn't work, try selecting a different category on the expense, saving the expense, then reselecting the correct category, and finally try exporting again.