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Once you create the graph:

sage: g = graphs.CompleteGraph(5)
sage: g
Complete graph: Graph on 5 vertices

The documentation of to_directed says:

sage: g.to_directed?
Signature:      g.to_directed(data_structure=None, sparse=None)
Docstring:     
   Return a directed version of the graph.

   A single edge becomes two edges, one in each direction.

   INPUT:

      * "data_structure" -- one of ""sparse"", ""static_sparse"", or
        ""dense"". See the documentation of "Graph" or "DiGraph".

      * "sparse" -- boolean (default: "None"); "sparse=True" is an
        alias for "data_structure="sparse"", and "sparse=False" is an
        alias for "data_structure="dense"".

   EXAMPLES:

      sage: graphs.PetersenGraph().to_directed()
      Petersen graph: Digraph on 10 vertices

So, this is what happens if you provide wrong input:

sage: g.to_directed('wrong input')
Traceback (most recent call last)
...
ValueError: data_structure must be equal to 'sparse', 'static_sparse' or 'dense'

Following what the documentation says:

sage: g.to_directed('dense')
Complete graph: Digraph on 5 vertices

You may also create a directed graph directly using DiGraph (various different type of inputs), see the documentation.

Once you create the graph:

sage: g = graphs.CompleteGraph(5)
sage: g
Complete graph: Graph on 5 vertices

The documentation of to_directed says:

sage: g.to_directed?
Signature:      g.to_directed(data_structure=None, sparse=None)
Docstring:     
   Return a directed version of the graph.

   A single edge becomes two edges, one in each direction.

   INPUT:

      * "data_structure" -- one of ""sparse"", ""static_sparse"", or
        ""dense"". See the documentation of "Graph" or "DiGraph".

      * "sparse" -- boolean (default: "None"); "sparse=True" is an
        alias for "data_structure="sparse"", and "sparse=False" is an
        alias for "data_structure="dense"".

   EXAMPLES:

      sage: graphs.PetersenGraph().to_directed()
      Petersen graph: Digraph on 10 vertices

So, this This is what happens if you provide wrong input:

sage: g.to_directed('wrong input')
Traceback (most recent call last)
...
ValueError: data_structure must be equal to 'sparse', 'static_sparse' or 'dense'

Following what the documentation says:of the method to_directed for undirected graphs says, you may instead write:

sage: g.to_directed('dense')
Complete graph: Digraph on 5 vertices

You may also create a directed graph directly using DiGraph (various different type of inputs), see the documentation.