This error message is typically associated with the TCPDF library, which is a PHP class for generating PDF files.
This error message is typically encountered when using the TCPDF library to generate a PDF file, and it indicates that some data (such as HTML or text) has already been sent to the browser before the PDF file is generated and sent.
To save a file generated by TCPDF to a specific folder, you can use the TCPDF "Output" function.