Could not determine JSON object type for type “Class”

In order to add an arbitrary non-primitive POCO to a JArray (or JObject), you must explicitly serialize it, using one of the overloads of JToken.FromObject():

_JArray = new JArray();

string[] amounts = repository.Amounts.Split('|');

for (int i = 0; i < amounts.Length; i++)
{
    _JArray.Add(JToken.FromObject(
        new AmountModel
        {
            Amounts = amounts[i],
        }));
}

return _JArray;

(Note also that I corrected the end condition in your for loop. It was i <= amounts.Length, which resulted in an IndexOutOfRangeException exception.)

Working sample .Net fiddle #1 here.

Alternatively, you could simplify your code with LINQ and JArray.FromObject() by projecting the string array to an AmountModel enumerable then serializing the entire sequence to a JArray in one call:

var _JArray = JArray.FromObject(amounts.Select(a => new AmountModel { Amounts = a }));

Sample fiddle #2 here.