It's locked in place. It's designed that way to prevent it from falling in during operation.
Here's how to remove it correctly. First, ensure the blender is off and unplugged. To remove the lid plug, you turn it counter-clockwise and then lift it out. Don't just pull—it needs that quarter-turn unlock first.
If it's truly stuck, check that the container isn't under pressure from hot ingredients. If you were blending something hot, allow it to cool first.
If that didn't work, never try to force it or use tools that could damage the plastic. The lid plug is also a measuring cup. If it's damaged, you need a replacement. Contact Vitamix for genuine parts.
Remember: You only remove the lid plug to use the tamper or to add ingredients. The main part of the lid must stay locked in place.