Preform tell me that the model is directly on the platform and that it is not the best way for a good print, but the problem is that I don’t see how to move my model on the z_axis
You don’t move it on the Z-axis because it cannot print in mid-air.
You either print it directly on the platform if you’re SURE that’s enough or add supports to it. Try the automatic supports first and then you can edit it to better suit your needs.
Yes you right and I know the model can not float in the air.
I thought that we can choose the distance between the plate and the model, but apparently not 
Thank you for your help 
In Supports-> Advanced Settings the is a entry for ‘Height above Raft’ and ‘Raft Thickness’. Both will raise the model above the build plate.
Ok thank you very much, I will try this 