And having bought the Magma Citadel castle, I was now able to activate it.

Hello lady Cammy , so at the end you sorted out i see !

Anyway wasn't any issue with the game , everything works as intended !

Please update us in case you need assistance again !

I wish you a good day and enjoy the game !

This castle cannot be installed if your castle is under siege. Lift the siege and all will be well.

Sometimes easier said than done :P  But it sounds like he wasn't sieged and just hadn't bought the castle skin.  

Please abstain from irrelevant comments  , what are you sayig is not included in the details of the poster  she wasn't seiged , and as you may note the poster was able to activate it .

Have a good day and enjoy the game !

I'm just sharing my personal experience. Isn't this a forum? I had such a problem - I wrote my solution. 

yes is a forum , but your solution wasn't for this case