I made a subclass Animals, a new subclass Crab, but the error remains. Message is: "method move in class greenfoot.Actor cannot be applied to given types; required: int; found: no argumens; reason: actual and formal argument lists differ in length.

Please show the actual codes as your description is a bit cryptic in nature. Use code tags to show your Crab class.

Is it possible to add printscreens? Copie-pasted it gives: public void act() { move(); turn(45); }

You need to add a number into the () after move, so it knows you far you want to move. The Animal class (which you can import) has a move method without parameters which uses a 'speed' value, the Actor doesn't have that. You can add an image of the code if you save a screenshot and upload it. Then you can use the img-tag to post it here. An easier way is to use the code-tags.

No -- the version of greenfoot does not matter. You must have the Crab class extend the Animal class to use the move method without a parameter.

...And the animal class should be the one the book is talking about, not one you created yourself leaving it blank.