Your problem here is that Object.keys(e) return an array of keys and not a single key.

If your object will always be the same (with one key) the you can get the key with : let element = Object.keys(e)[0]

I've decomposed the code with 2 main parts.

• Getting the key of the current item (with const key = Object.keys(current)[0])
• Checking if the newObject has the key
• If yes : updating the grade and counter value with the current object
• If no : adding the current object to the key

const arr1 = [{
    '1': {
      grade: 1.3,
      counter: 2
    }
  },
  {
    '1': {
      grade: 2.8,
      counter: 2
    }
  },
  {
    '2': {
      grade: 4.5,
      counter: 1
    }
  },
  {
    '2': {
      grade: 2.4,
      counter: 1
    }
  }
]

const newObject = arr1.reduce((newObject, current) => {
  const key = Object.keys(current)[0]
  const associatedObject = newObject[key]
  if (associatedObject) {
    associatedObject.grade += current[key].grade
    associatedObject.counter += current[key].counter
  } else {
    newObject[key] = current[key]
  }
  
  return newObject
}, {})

console.log(newObject)

Try this

let arr1=[{'1': {grade: 1.3, counter: 2}},{'1': {grade: 2.8, counter: 2}},{'2': {grade: 4.5, counter: 1}},{'2': {grade: 2.4, counter: 1}}];

arr1 = arr1.reduce((acc, obj) => {
    let [k, v] = Object.entries(obj)[0];
    if(!acc.hasOwnProperty(k)) acc[k] = {grade:0,counter:0};
    acc[k].grade += v.grade;
    acc[k].counter += v.counter;
    return acc
}, {});

console.log(arr1)

I would go for:

arr1.reduce((o,n)=>{
  Object.keys(n).forEach((key)=>{
    if(!o[key]) o[key]={grade:0, counter:0};
    o[key]["grade"] += n[key]["grade"];
    o[key]["counter"] += n[key]["counter"]; 
  });
 return o;
},{});

Step1: reducing over the array, acc = {}

Step2: iterating over every key of the object at the current index

Step3: summing up the according properties

You forgot to iterate over the keys.

Comment: As a pons asinorum I use for array.reduce() the variable names:

• o meaning old value (=acc=_accumulator)
• n meaning new value.