Apple's calculator can't do math.

In the Apple calculator app, switch to Scientific mode. Enter an integer or decimal fraction and click the +/- button to change it to a negative number. Then click the e^x button and there is no error, a number is produced. I can raise e to the power of -.5 and get 0.61.

I can open the WolframAlpha web page. I enter x=-.5; e^x and get the same (if rounded) answer as above.

Yes, I can confirm that the Calculator app on macOS High Sierra 10.13.6 does indeed report "Not a number" when provided a negative number for e^x. Makes no difference whether RPN mode is selected. This is a bug and there is no remedy unless you use another calculator tool. Apple won't be fixing this.

For instance, the UNIX bc tool, when used in the macOS 10.13.6 Terminal application, works just fine for negative exponent usage whether integer or decimals: