Here are some reasons why you get the message “Unable to find main dictionary. Could not start speller.” or “unable to find main dictionary could not start spelling” in AutoCAD (or vertical) or AutoCAD LT when you try to add or change text or make a spell check.
You have deleted some DCT files in your support folder and might need to restore them from the CD/DVD alternatively make a repair of the installation.
Make sure to install dictionaries as feature if you select a custom installation. Even if you don't need multi-language dictionaries you will get an annoying error message every time you want to create or edit text. "AutoCAD Message - Unable to find main dictionary. Could not start speller."
If AutoCAD is already installed use Windows Control Panel>Programs and Features and find the AutoCAD installation and select Change or “Add or Remove Features” and make sure "Dictionaries" is un-checked and proceed with the update of the program.
You might have deleted the path to the location of the DCT files in OPTIONS. You can check that you have the path to where the DCT file are among the paths in OPTIONS>Files>Support File Search Path. If it is not there add it and restart the application. Another way is to restore the profile but then you might loose a lot of your settings that you have customized.
Here are some of the DCT files that comes with AutoCAD:
This post was made thanks to an email question I got on this problem and I wanted to give the solution to others that might run into this.