I have got a problem building PVP server

I have got a error message about codes.But i dont know how to coding in visual studio.But i wanna play with my friends to entertain with each other in us PVP Server
Can somebody help to solve this problem ?
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If you don't understand basic coding in c# it's can be really hard to create a good pvp server, but what you want do? What is your end point?

change ToSbyte in ToInt16