It's great to hear from you again, Diane18!
QuickBooks Desktop's payroll service is a great asset for tracking payroll calculations, including liabilities, as I'm sure you know. It sounds like you may have a unique situation here and I want to make sure you get the details right. I'll go over how payroll liabilities work in QuickBooks Desktop and what you can do for your situation with the union dues needing to have taxes added on.
In QuickBooks Desktop, the payroll system keeps things simple by housing all of your payroll features in one place: the Payroll Centre. This means even your payroll liabilities are handled through that area. To do this, go to the Pay Liabilities tab, then click the Pay Liabilities button just below the Pay Taxes & Other Liabilities heading. This function doesn't include the option to add taxes. You can read more about the process here: Pay or adjust payroll liabilities
With the message you've mentioned you're getting, it sounds like you may be trying to pay the liabilities outside of the payroll module. Directly attributing funds to certain kinds of accounts, such as sales tax accounts and the payroll liabilities account, can impact the calculations recorded since the program wants to do it through the appropriate modules. While it can be done outside the modules, it's important to be mindful of how it will impact your books.
As I mentioned in the beginning, your situation with the union dues needing to have taxes added onto the amounts calculated sounds unique. Typically deduction and contribution items don't have an additional tax, which is why QuickBooks Desktop isn't showing you the options to do so.
In light of that, I recommend speaking with a QuickBooks-savvy accountant to figure out how to record it in QuickBooks. They should also be able to give you advice on whether this item should have the taxes you've mentioned. I believe I may have left you with this information before, but to make sure you have it, here's our page where you can find an accountant certified in QuickBooks Desktop: Find an Accountant
I hope that helps. Let me know if you have any other questions about how to navigate or use QuickBooks. Have a great rest of your day!