According to an answer on Unity Answers, you need to put the scripts used for building into a folder named Editor (specifically, ./Assets/Scripts/Editor). Here's a full working build script:

using UnityEditor; using UnityEngine; class BuildSrcipt { static string[] scenes = { "Assets/Scenes/MyScene.unity }; static string name = "MyGame"; [MenuItem("Build/Build WebGL")] static void BuildWebGL() { BuildPipeline.BuildPlayer(scenes, "./" + name + "_Web/" + name, BuildTarget.WebGL, BuildOptions.None); } [MenuItem("Build/Build Windows")] static void BuildWindows() { BuildPipeline.BuildPlayer(scenes, "./" + name + "_Windows/" + name, BuildTarget.StandaloneWindows64, BuildOptions.None); } [MenuItem("Build/Build Linux")] static void BuildLinux() { BuildPipeline.BuildPlayer(scenes, "./" + name + "_Linux/" + name, BuildTarget.StandaloneLinux64, BuildOptions.None); } [MenuItem("Build/Build All")] static void BuildAll() { BuildLinux(); BuildWindows(); BuildWebGL(); } } 

According to an answer on Unity Answers, you need to put the scripts used for building into a folder named Editor (specifically, ./Assets/Scripts/Editor). Here's a full working build script:

using UnityEditor; using UnityEngine; class BuildSrcipt { static string[] scenes = { "Assets/Scenes/MyScene.unity" }; static string name = "MyGame"; [MenuItem("Build/Build WebGL")] static void BuildWebGL() { BuildPipeline.BuildPlayer(scenes, "./" + name + "_Web/" + name, BuildTarget.WebGL, BuildOptions.None); } [MenuItem("Build/Build Windows")] static void BuildWindows() { BuildPipeline.BuildPlayer(scenes, "./" + name + "_Windows/" + name, BuildTarget.StandaloneWindows64, BuildOptions.None); } [MenuItem("Build/Build Linux")] static void BuildLinux() { BuildPipeline.BuildPlayer(scenes, "./" + name + "_Linux/" + name, BuildTarget.StandaloneLinux64, BuildOptions.None); } [MenuItem("Build/Build All")] static void BuildAll() { BuildLinux(); BuildWindows(); BuildWebGL(); } }