raised as issue

Translation added to https://www.codewars.com/kata/fluent-calculator-1
Do I un-publish this Kata now to close it?

They are the same thing
You need to clear s because you are returning this which is not necessary if you return a new object without modifying s

Will do

Actually no error, because this is not tested at all.

That's another thing the description did not specify and which there are no tests for. aka:

FluentCalculator calc = new FluentCalculator();
FluentCalculator calc1 = calc.Seven.Plus.Eight;
FluentCalculator calc2 = calc.One.Plus.Two;

Actually it's not clear expression while calling Result but obtain a new object while doing every operators

Duplicate of https://www.codewars.com/kata/fluent-calculator and https://www.codewars.com/kata/fluent-calculator-1. You should make a translation to those katas instead.

It's not mentioned in the description that calling Result should clear the stored expression so far.

calculator.One.One //Should Error

Should it be a CompileTimeError or RuntimeError?

I suggest updating the description to "Create a simple calculator that given a string of operators () + - * / and numbers separated by spaces returns the value of that expression".
It caused some confusion initially, given that the Sample Test Cases don't reflect the submission ones.

I'd consider adding these test cases to the sample ones:
[TestCase("2 / 2 + 3 * 4.75 - -6", ExpectedResult = 21.25)]
[TestCase("(2+2)(4)", ExpectedResult = 16)]
[TestCase("12-1", ExpectedResult = -12)]
[TestCase("((80 - (19)))", ExpectedResult = 61)]
[TestCase("12 * 123 / -(-5 + 2)", ExpectedResult = 492)]