This is one of those times when you are about to throw the waste paper basket rather than throw something in the waste paper basket. You attempt to embed a tablix within a tablix and when you preview, get the error “The Tablix includes a table header or column with Fixed Header set to true”.
Check your row and column groups by clicking on the name of each group in the Row Groups and Column Groups sections in Report Designer or Report Builder. Now check the properties, look for the Fixed Data property under the Other Property category. If this is True, set it to False and retry your report.