PostEffect is an abstract base class for post-processing effects, such as BloomEffect and ColorCorrectionEffect. They change how the world looks after it has been rendered. They do not affect GuiObjects. Objects of this kind should be parented to the Lighting or the Workspace.CurrentCamera in order to work.
It should also be noted that some post-processing effects will work differently or not at all when Roblox is set to a low QualityLevel (or EditQualityLevel in Studio). On some low-end devices, faster rendering algorithms may be used. By default, these quality settings are set to Automatic, so if you aren't seeing post-processing effects you should check Roblox's settings under the "Rendering" section. It may be necessary to override the automatic behavior temporarily in order to preview post-processing effects.