If you want to sum a manually selected (highlighted) cellrange:
1. Click on target cell
2. click on SUM icon on the toolbar,
3. highlight the desired cellrangerange with mouse,
4. hit Enter.
Tibor Kovacs, Hungary; AOO4.1.2; LibreOffice4.4.7 and Portable LibreOffice5.1.6 / 5.2.3 versions on Win7x64Prof
Please, edit the topic's initial post, and add the word "[Solved]" at the beginning of the subject line - if your problem has been solved.