ASP.NET 3.1 Core : System.FieldAccessException

I am getting exception in my Asp.Net core 3.1 application as below

System.FieldAccessException: Cannot set initonly static field 'Field' after type 'TestReadOnly' is initialized.

Test.GetType().GetField("ImmediateTest", BindingFlags.NonPublic | BindingFlags.Static)
        .SetValue(Test, Timeout.Infinite);