If you have a panel that is missing, or that you cannot hide. Anything wrong with the UI layout really, you can use Window - Reset Layout to restore the application to the default settings.
If ever that is not sufficient, maybe there is a corrupted layout file. These are managed by the third party library we use for our UI and sadly we have to control over them.
Close the application and delete all of the *.lyt files from the local storage folder.
By default this is %LOCALAPPDATA%\Devolutions\RemoteDesktopManager.
Post a new topic is this doesn't resolve your issue.