Unity NetworkVariableでstringが使えないので FixedString32Bytesを利用してみる。

ArgumentException: Type System.String is not supported by NetworkVariable1. If this is a type you can change, then either implement INetworkSerializable or mark it as serializable by memcpy by adding INetworkSerializeByMemcpy to its interface list. If not, assign serialization code to UserNetworkVariableSerialization.WriteValue and UserNetworkVariableSerialization.ReadValue, or if it's serializable by memcpy (contains no pointers), wrap it in ForceNetworkSerializeByMemcpy1.

という感じで
NetworkVariable<string>が使えなそうで

NetworkVariables | Unity Multiplayer Networking

NetworkVariables are a way of synchronizing properties between servers and clients in a persistent manner, unlike RPCs and custom messages, which are one-off, p...

docs-multiplayer.unity3d.com

Unity fixed　string types: 

FixedString32Bytes, FixedString64Bytes, FixedString128Bytes, FixedString512Bytes, and FixedString4096Bytes (which are serialized intelligently, only sending the used part across the network and adjusting the “length” of the string on the other side to fit the received data).

ということで NetworkVariable<FixedString32Bytes>に変更しました。