Non-linear solver for LibreOffice?

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.