Your code is correct, you just have a minor typo with your brackets. [] instead of ().

A small hint to optimize the performance of your code:

You can actually skip the whole if condition when increasing your i by 7 instead of 1.

function returnSevens(max) {
    let sevenarr=[];
    for (let i = 0; i<max; i += 7) {
         sevenarr.push(i);
    }
return sevenarr;
}

Push is a method. so you need to use () instead of []

function returnSevens(max) {
    let sevenarr=[];
    for (let i = 0; i<max; i++) {
        if (i % 7 === 0) {
            sevenarr.push(i);
        }
    }
return sevenarr;
}
console.log(returnSevens(14))

You already have the answer to your question, but I'd suggest to do some adjustments.

1. Declare the array you'd like to return as const, not let. So it wont be changed by accident.
2. Instead of running the loop with step 1 and testing each value, you can loop it with step 7 and skip testing at all. This code is much faster. It is not very important for small values, but for huge quantities and complex test operations, it might be significant.

function returnSevens(max) {
  const sevenarr = [];
  for (let i = 0; i < max; i+=7) {
    sevenarr.push(i);
  }
  return sevenarr;
}
console.log(returnSevens(14));