I did read this, and it says an array of subarrays or of tuples or a string

It doesn't say that you can only return a string, and when I tried to return an array of tuples, it results in errors like this one with the tests:

Test Failed
Expected is <System.String>, actual is <System.ValueTuple`2[System.Int64,System.Int64][3]>
Values differ at index [0]
Expected: '['
But was: (1, 1)

It expects a string in the specific format shown in the tests and returning an array of tuples as the description specifies as an option doesn't work

It is said:
"We will return an array of subarrays or of tuples (in C an array of Pair) or a string.".
Did you read that ?
Furtherrmore you have the following "Sample Tests":
[Test]
public void Test01() {
Assert.AreEqual("[[1, 1], [42, 2500], [246, 84100]]", SumSquaredDivisors.listSquared(1, 250));
}
[Test]
public void Test02() {
Assert.AreEqual("[[42, 2500], [246, 84100]]", SumSquaredDivisors.listSquared(42, 250));
}
[Test]
public void Test03() {
Assert.AreEqual("[[287, 84100]]", SumSquaredDivisors.listSquared(250, 500));
}
}

In C#, although the kata's description says that you can return an array of tuples/subarrays, you can only return a string and need to manually format it to look like the one in the example.