You will need to create a GL Account in QuickBooks Online called "Sales Tax Payable". The "Taxes as Line Items" module will treat Sales Tax lines as any other service item line.
For more information see our article on Sales Configuration and look at the Service Item Source:
Service Item Source: Specifies how service items (invoice lines with no ItemID) will be created in QuickBooks Online. Mobius Connect requires creating “Service” items in QuickBooks Online to successfully map GL Accounts for invoice lines that have no ConnectWise PSA products. This option tells the integration how you would like those products created.
Once the new GL Account is set up in QuickBooks Online the issue will resolve.