Hello Mfmedia India,
Happy to chime in!
The best way to view Storyline 360 courses designed for wide screens on mobile phones is to use the landscape orientation. There isn't a way to remove the white spaces in your screenshot, but using the landscape orientation would greatly reduce this space on screen. Here's more information on how the Responsive Mobile Player works:
If your courses will be viewed mostly on mobile phones that use the portrait orientation, consider changing the slide size of your project to match the resolution of the devices.