Em portugues:

No KDE Neon e KDE. instalamos o synaptic (synaptic pakage manager) e na pesquisa digitamos libatomic,irá aparecer varios itens,vamos instalar o libatomic1 e depois baixamos e instalamos o dropbox.O dropbox irá funcionar normalmente.Fico grato por postar.

At english:

Problems with dropbox in KDE Neon and KDE.

In KDE Neon and KDE. we install synaptic (synaptic pakage manager) and in search we type libatomic, it will pop up several items, we will install libatomic1 and then we will download and install dropbox. The dropbox will work normally. I am grateful to post.

@fabserrano - thank you!

Tried so many ways to install Dropbox and it never worked!

Installed Synaptic, then libatomic1 as per your tip and then Dropbox via Discover and everything is good!

Thank you!