<p>My problem is that I have used a 3D printing machine from the University and found out that the cover for the car was not smooth even after using sanding paper and painting it.</p>

<p>What material would work best to print  the cover of the Cyber truck. I want it to be light and smooth.</p>

<p>I have to print it from any online companies that have this service here in Germany.</p>

I have 3D printed models which were then sanded using progressively finer grades of sandpaper, terminating with wet sanding using micromesh to 12000 grit. The result was smooth and shining without any coating applied.
If your original results were not acceptable, the process may have been flawed and should be re-considered for technique.
For your purposes, as a body for a radio controlled vehicle, you'll want to consider something that can manage an impact reasonably well. ABS is going to be less expensive and provide some energy absorption but will have layer lines that require sanding and finishing. Layer thickness plays a substantial part in providing for good results and a smooth finish. I used 0.100 mm layers to get optimum smoothness.
You could request your model to be created in nylon using the SLS method, but the surface will be granular and would also require sanding to accomplish a smooth finish.
SLA or MSLA resin printed models will provide a very smooth surface, but the material is brittle and may crack during "on-road" use. You may find a printing service which offers to create using a more flexible resin, but you'd have to request that or confirm the selection when placing the order.