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Most probably you did not include css/js file for fixedHeader in your code. please include the link https://cdn.datatables.net/fixedheader/3.1.5/css/fixedHeader.bootstrap.css in head section of your page and this https://cdn.datatables.net/fixedheader/3.1.5/js/dataTables.fixedHeader.min.js in your script section. It may solve your problem.
Source:stackexchange.com