its happening sometimes lately. Seems server problem or your game is too big, maximum 100mb including all resources
A server side error? If you click the ‘see logs’ button you can read more detail about the nature of the error.
Unfortunately it is happens very often with the online build service and as xisco mentioned currently the upload is limited to 100Mb. Also make sure your version of GDevelop is up to date and you reference resources within the project folder only. If you want to only test your game on mobile, better to try the preview over wi-fi option or export HTML5, upload it to a host like itch.io and open it on your mobile in Chrome browser in full-screen. Since the online build service uses Cordova which ‘only’ wrap your game in to Chromium, you should get the same result in your Chrome browser on your mobile. Personally find it more reliable for testing.
OK ! So my bad! 
! I was just curious with the buttons on properties of my game ! So being curious I changed the package name for my game! Cause there was literally game name written so I thought I had to change it to my game name so I changed it and problem occured!! I corrected it by changing it to what it was before and this fixed the error! BTW thanks for trying to help!! And thanks for the info!! and I was just trying to put an icon for my game when I messed it up!!
Interesting. Maybe, @4ian would like to look in to this, I think changing the package name should not break the build. However, at least we slowly collect a nice list of things that can break the build 
Thanks for sharing.
What was the package name you’ve entered?
I think i gave it com.mygame.mad land survival or com.mygame.madlandsurvival ! Not sure about that!
Spaces are not allowed, might be why you got an error. Though GD should not allow you to set such a package name.