I don’t know how to remove it.
It is very difficult to provide help with your question as it is. Please provide more information: what where you doing at the moment; whether you were working in Calc or in Writer, etc; what kind of input there is in your document (e.g., text, numbers, links, etc.), and, in general, anything that can help the community understand what you experience. Also, check What information should I include in a question in addition to OS and LibreOffice version?