+0000TCPDF ERROR: Some data has already been output, can't send PDF file

I have a PDF account template and when I try to Print to PDF I get the following error

+0000TCPDF ERROR: Some data has already been output, can’t send PDF file

I am able to view and edit the template with no issue. Is there a PHP that needs editing? If so where/what? ALso what log do I check?