there’s confusing parsing problem going on in your example. -2.68e15^0.2 is parsed as -(2.68e15^0.2) which exists. Try (-2.68e15)^0.2 and it’ll error. (-2)^0.2 is a much easier example. Fractional exponentiation is defined by the complex log function. The easiest example to think of here is that (-1)^0.5 = sqrt(-1) = im. So in this domain it cannot be guaranteed to be a real number. It gets even worse because exponentials/logs actually don’t always have unique solutions, so you have to take principle branches and stuff like that. Here’s a blurb:
But long story short, negative numbers to exponents <1 are generally defined as complex valued (via applying the complex exponential defined via the principle branch of the complex log), which gives this trouble.