unity major version 51 is newer than 50, the highest major version supported by this compiler.

如題，

unity build中出現

unity major version 51 is newer than 50, the highest major version supported by this compiler.

代表你的java 版本有問題

請先移除java所有版本，重新下載或是保留java 7即可

並注意修改path與classpath環境變數中內有java路徑的部份