Posting group for FA should be only possible to change from the FA Depreciation Book Line page/table. You can open it form FA card by pressing Depreciation Books button.
From the error it seems that You are trying to change this FA Posting Group directly in Journal line or document line. I wouldn't recommend it - but as always depends on the business reason.
From your question, difficult to understand what exact action you are doing, can you please share screen shot/ more information - so that it will help to answer your query.
Thank you for your reply! To ensure a great experience for everyone, your content is awaiting approval by our Community Managers. Please check back later.