I was under the impression that HiddenAssets Folder is ignored during a build. Is this not the case or are there special rules for Editor scripts. Assets/HiddenAssets/....ManagerEditor.cs(14,55): error CS0246: The type or namespace name 'Editor' could not be found (are you missing a using directive or an assembly reference?)
I haven't used it myself, but the documentation spells the folder name with a space like so: Hidden Assets Other folders are case and space sensitive as well, so I assume that may be the issue.
Editor scripts must be located in an "Editor" folder. In order to exclude folders from Unity entirely, you can add the ~ symbol at the end of the folder. It's both explained in the link Xarbrough posted. You can see this in Unity's ScriptableRenderPipeline repository for example, where they use it for the "Documentation~" folder: https://github.com/Unity-Technologi...unity.render-pipelines.high-definition-config