Text control is very limited in after effects. There is no functionality in the Character Panel to achieve a strikethrough at the moment.
Using Illustrator
For better text control I'd suggest create your text in illustrator instead:

and import the illustrator file into after effects afterwards:

Note: Editing the text in illustrator is still possible.
Using a shape layer
In order to create a simple stroke above your Text Layer:
- Enable the Pen Tool (G)
- With no layer selected, click into the Viewer (this will create the 'first point' of the stroke)
- Hold down Shift to make sure that the line, which you are going to create is 'straight'
- Click into the viewer again to set the 'end point'
This will create a new Shape Layer. If the layer is selected you can adjust the 'stoke color' and the 'thickness' in the Tool Bar:

Another way is to roll out the Properties of the layer by clicking on the tiny triangle > left beside the layers name, go to Contents > Shape(Number) > Stroke(Number) to adjust the 'color' and the 'width'.
However a shape layer provides much more control in terms of animation:
- Add a Trim Paths property
- Create a first keyframe by clicking on the stopwatch of the
End
value and set it to 0
- Move a few frames forward
- Create a second keyframe and set
End
value to 100

Related: https://graphicdesign.stackexchange.com/questions/49643/how-to-use-after-effects-trim-path-with-an-illustrator-path/49975#49975
Result
