Unfortunately this is tricky to figure out without seeing your workbook.
Does the URL look as your would expect when you publish? Typically when you click "Test link", it's only going to test one value, so if other values are null in your data source, that could be some of the problem.
I'd also just like to clarify: Are <Tableau_Url> and <car_no> both fields in your dataset? Or is <Tableau_URL> the URL of the dashboard you are publishing? You could be running into issues with that as well.