Minecraft Launcher Doesn't Let Me Start The Game, There Is A Error Message.

Put the summary of the bug you're having here

What I expected to happen was that the game would launch.
Describe what you thought should happen here: So what I thought would happen was when I pressed play, the game would launch.

*What actually happened was that a error message popped up.
Describe what happened here: When I pressed launch, the game wouldn't launch and it came up with an error message within the launcher.

Steps to Reproduce:
1. Turn on the launcher.
2. Press play and wait game to load.
3. After a second the error should pop up in the launcher.