If the statistic as a parameter in ratio function is '', statistic.length is 0. And this causes mathematical error because something cannot be divided by 0. If you write some exceptional code like following, it will work well.

function ratio(statistic) {
   let charcount = 0 ;
   for ( let i = 0 ; i < statistic.length ; i++){
      if ( statistic[i] == 'a' ){ 
         charcount += 1 ;
      }
   }
   return (statistic == ''? 0 : Math.round(charcount/statistic.length * 100)) ;
}

when you have empty string or null string, the value of statistic.length will be 0. So you are missing of this check in your code that is why you are getting NaN (Not a Number).

function ratio(statistic) {
 let charcount = 0 ;
 for ( let i = 0 ; i < statistic.length ; i++){
   
   if ( statistic[i] == 'a' ){ charcount += 1 ;}
    
}
// Corrected code here 
return statistic.length == 0 ? 0 : Math.round(charcount/statistic.length * 100) ;
}
console.log(ratio('abababaabaaa'));
console.log(ratio(''));
// 67
// NaN <---- here should be 0

Try:

function ratio(statistic) {
  let charcount = 0 ;
  for ( let i = 0; i < statistic.length; i++ ) {
    if ( statistic[i] == 'a' ) { charcount += 1; }
  }
  if (charcount === 0) return 0; // check if there were no a's in the string, if so return 0
  return Math.round(charcount/statistic.length * 100) ;
}
console.log(ratio('bbbbbbbaaa'));
console.log(ratio(''));

To prevent from having to divide by 0, we just check if no a's were found and if so return 0 then and there.