I play using my laptop- HP ENVY x360
Windows 10

I use Nexon Launcher.
I signed in through my gmail account. (Not sure if linking your account with gmail matters or not)
Maplestory will not launch. (Seeing the Nexon Security pop up is such a tease)
I've tried repairing the Nexon Launcher and Maplestory client.
I've tried uninstalled and then reinstalling both the Launcher and Maplestory,
I still can not open the game. (AHHHHHH!!!!)
At this point I was ready to give up... If this sounds something like your problem then continue reading.

After some research, I've come across a youtube video (Thanks to VEGAN) with a fix to run Maplestory.
Fix:
1. Click 'Windows + R' to open up the Run command window.
2. Type: "REGEDIT" to open your computer's registry files.
3. Click 'CTRL + F' to quickly locate the registry file we need.
4. In the "Find" window type "soScreenMode" and double click to open it.
5. Change the Value Data to "3" Hexadecimal and hit "okay".
6. Now, try launching Maplestory again.
7. If successful, it should now run- but in a windowed screen.
8. Once you're finally inside the game, click 'ALT + Enter' on your keyboard to run the game in fullscreen.

Hope this helps. Don't give up~