Best thing is to watch 

Video 10 August 2020 - Stoyan Shopov  for the given exam and compare your code step-by-step until you find all problems.

One Test case didn't work, giving 90/100 from judge:

        [Test]
        [TestCase(10, 15)]
        [TestCase(25, 50)]
        public void WorkWhenTheBatteryIsLow(int battery, int batteryUsage)
        {
            var newrobotmanager = new RobotManager(10);
            var db = new Robot("Robby", battery);

            Assert.Throws<InvalidOperationException>(() =>
            {
                //newrobotmanager.Work(null, "Charge", batteryUsage);
                newrobotmanager.Work("Robby", "Charge", batteryUsage);
                // you need to use the robot's name,
                //otherwise you will fall in the wrong Exception case for invalid Robot
            });

        }

Here comes the tricky part when you have 90/100, it's most certainly one of the tests that check Add/Remove/Charge/Work. The problem is not the cases themselves, they may be written correctly, but the data that you are using is not valid for judge. See discussion on Проблем с Unit Testing - C# OOP Exam 16 August 2020

where iGeorg1ev has thankfully found the problem.

Best,