Hi Tom and Dan, thanks for the answers.
To set a context, I am going to explain my goal with the scale bar. I work with 3D model and my ultimate goal is to have my scale bar related to my 3D model depending on my zoom and my distance from the model or a plan that I created in the same plan as the model as you can see on the picture for example.
The problem, I think, is that Arcgis Pro use the vertical axe or the elevation from the ground to define the scale of the scene, as we are supposed to work on map (more like 2,5 D and not real 3D) and it seems more logical.
Other way, the Convert To Graphics tool that you told me about can work as i can create my own scale bar and adjust it to the model as I can measure distance on the model and the size of the paper in the layout.
If you know how to create a scale bar related to a plan or a model, I will be happy to know about it.
Otherwise thank again for the fast answer !