The FAQ https://www.codingame.com/faq specifying the environment for C# is a bit misleading. It says
C# 7.0 (Mono 6.4.0, .NET 4.7)
but Mono 6.4.0 only implements a subset of C# 7 (see https://www.mono-project.com/docs/about-mono/languages/csharp/)
I would suggest to either remove the C# 7.0 part of this fact and leave only the Mono part, or correct it for C# 7.0 (Partial)