en.javascript.info/1-js/03-code-quality/02-coding-style/1-style-errors/solution.md

You could note the following:

function pow(x,n)  // <- no space between arguments
{  // <- figure bracket on a separate line
  let result=1;   // <- no spaces to the both sides of =
  for(let i=0;i<n;i++) {result*=x;}   // <- no spaces
  // the contents of { ... } should be on a new line
  return result;
}

let x=prompt("x?",''), n=prompt("n?",'') // <-- technically possible,
// but better make it 2 lines, also there's no spaces and ;
if (n<0)  // <- no spaces inside (n < 0), and should be extra line above it
{   // <- figure bracket on a separate line
  // below - a long line, may be worth to split into 2 lines
  alert(`Power ${n} is not supported, please enter an integer number greater than zero`);
}
else // <- could write it on a single line like "} else {"
{
  alert(pow(x,n))  // no spaces and ;
}

The fixed variant:

function pow(x, n) {
  let result = 1;

  for (let i = 0; i < n; i++) {
    result *= x;
  }

  return result;
}

let x = prompt("x?", "");
let n = prompt("n?", "");

if (n < 0) {
  alert(`Power ${n} is not supported,
    please enter an integer number greater than zero`);
} else {
  alert( pow(x, n) );
}