In 'Tutorial: Duplicant Movement' is a little typo. it says 'Duplicants can only climb two tiles high and cannot fit into spaces smaller that two tiles'. 'That' should probably be replaced with 'than'.
I can only read English, so I don't know if this typo has occurred in other languages.
This exists in development build Q3-336724.
A simple look into the 'Database', then 'Tutorials', then 'Tutorial: Duplicant movement' should do the trick.