Windows 8/8.1/10 "Failed in finding proper screen mode for Gr2D"

A few things I did to fix this problem was to run Setup.exe in the installation folder to make sure the monitor settings for the game are working for the computer, then for me I think changing my monitor refresh rate from 59hz to 60hz is what fixed that error specifically, I changed this by right clicking the desktop and clicking Personalize, then display, then Adjust resolution, then Advanced Settings, then the Montor tab has the refresh rate setting.

Edit:
If you're monitor has a higher resolution than 1920x1080 then lower it to 1920x1080 while running MapleLegends, I've read about people with higher resolution monitors having issues running old version of Maplestory. If that doesn't work then change your resultion back to what it was and install and run GRU to force your computer to have a lower resolution, but chances are you have a monitor resolution 1920x1080 or lower so this will not apply to you. Also make sure you're running a standard resolution and not a custom one for example run MapleLegends in either 800x600, 1280x720, 1600x900, or 1920x1080, and pick the biggest one that your monitor supports, but this should already be set by default.

 

The last thing I can think of is try changing the game exe or shortcut to run in Compatibility mode using Windows XP (Service Pack 2) and if if that fails then Windows XP (Service Pack 3).