First make sure that processing plugin is activated. Go to plugins/install and manage plugins and look for processing. Make sure it is activated. It should look like this:
You must have a Processing menu. Choose toolbox. In the toolbox, go to the bottom and choose advanced interface. I think you are trying to use SAGA reclassifying tool. I personally prefer the GRASS reclassify tool. In order to work with this tool you must create a text file with the classification rules. It must be like this:
Once you have the classification rules file save it as a txt file. If you are using Windows you can create this file with Notepad. Now type r.reclass in the filter box as shown in the image and choose
Once you select
r.reclass you must see this dialog box:
You must choose the input raster, the file that contains the classification rules and also the output file. That should do it.