download, unzip and copy the sqlite3 folder to your project root so that it becomes a sibling of cms, sapphire and co.
either use the installer to automatically install SQLite or add this to your _config.php (right after "require_once("conf/ConfigureFromEnv.php");" if you are using _ss_environment.php)
If you're trying change a field constrain to NOT NULL on a field that contains NULLs it aborts the action because it might corrupt existing records. In order to perform the action anyway add the URL parameter 'avoidConflict' when running dev/build which temporarily adds a conflict clause to the field spec.