When you choose "Insert Field..." from the Layout menu, ClarisWorks puts the field at the bottom of the body, below any other object, in the current layout. If the body line is down near the extent of the maximum layout size and there is any object (field or text or graphic) near the body line, then there will not be enough room to insert the field and you will get the message 'Some fields do not fit on the standard layout. Move existing fields to create space and then insert remaining fields in layout mode.'
The solution is to free up space at the bottom of the layout and then insert the fields.