According to an answer on [Unity Answers][1], 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();
 }
 }


 [1]: https://answers.unity.com/questions/576746/build-error-the-type-or-namespace-name-unityeditor.html