Adding Shift as a Button

I tried using Shift in my 2D platformer to make my character run. However, Unity tells me that Shift isn’t set up in the Input Manager. I tried setting it up, but whenever I type Shift as a positive button, it doesn’t save the input. How do I set up shift as an input button for running?

Input.GetKeyDown(KeyCode.LeftShift); or
Input.GetKeyDown(KeyCode.RightShift);

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