You've tried to install the "Solver for Nonlinear Programming" for LibreOffice on Ubuntu but still can't use the new Solver when you open the dialog from

Tools > Solver...

Check you have the error I've encountered by looking up the nonlinear solver under

Tools > Extension Manager...

"Error: The status of this extension is unknown".

The plugin was written for OpenOffice, not LibreOffice. You have to fix this by uninstalling LibreOffice and then installing OpenOffice.