have you tried login first and then export?
Can you check the syntax of the line.
tabcmd export "MonthlySales/BrandSalesAnalysis?.BUSINESS%%20UNIT=US"
--fullpdf -f "C:\Tab\US.pdf"
Can you confirm the field you are filtering on?
Is it .BUSINESS UNIT
(All in CAPS?)
I think that case matters with the URL, so this needs to match the Field name in Tableau.
Also, a "space" is %20 not %%20
And is there meant to be a leading period (.)?
I would start with testing the export without a filter and ensuring you get output then, and then add the filter and testing.
Hope this helps