What are you trying to do? Your AMO command tells me that you have selected the phone with Prime Line = 5135, and you are trying to REVISE the configuration of the Secondary Line "4935" which is currently button # 10 on that same phone. The message is trying to advise you that "4935" is not already configured on that Line Key.
If you are trying to ADD that station "4935" to the phone with Prime Line "5135", then you would perform ADD-KCSU.
This type of configuration requires some previous knowledge and skill, as the Line Key must first exist in AMO TAPRO.
If you are using ComWin/Expert Access to perform these AMO commands, I suggest that you use the Com-Edit function. This function can be "called" by pressing CTRL + TAB keys.
When the ComWin-Edit window appears, select AMO KCSU in the "Command" field, then use your mouse and click the GREEN "Command" Link. Here you will see handy information about how to use AMO KCSU.
If the information is in German, click "Options -> Settings", then change the Language to English, and try again.