I bouth a new mavic platinum my first drone and I tri to flight it and the controler tel me mag interferance I don't no wy ??

Hello and good day Michel Rajotte. I am sorry for the trouble and thank you for reaching out. Have you tried to fly to a different location to see if the issue will still persist? With regards to the interference you have experienced on you previous flight, most of the cause of the interference would be any wireless signal that interfering signals from the remote controller to the said drone. In addition it is recommended to calibrate the compass and the IMU if you go to fly your drone to a new location further away from the past location. In addition let me provide you additional requirements for proper compass calibration. Do not calibrate your compass where there is a chance of strong magnetic interference, such as magnetite, parking structures, or steel reinforcements underground. Do not carry ferromagnetic materials with you during calibration, such as keys or cellular phones. Do not calibrate beside large metal objects. I hope these helps. Thank you.

Are you powering it on while its sitting on a metal table?

Where / What are you trying to takeoff from?  

Any ferrous metal (aka steel) within 1-meter of takeoff spot?  

Are you standing close to Mavic Pro P and possibly have something magnetic on you?

Have you tried moving to different takeoff area?

Any messages about Mavic Pro P compasses needing to be calibrated?

Have you looked at Mavic Pro P's compasses magnetic field strength levels under Advanced menu of GO-4 App?  Both should have short green bars.

Take it outside in a wide open area away from magnetic material and calibrate the compass