The online accounts in system settings are completely non-functional. Pressing the + to add a new account does nothing.
I faced a similar issue on Gentoo. However, I by chance found a workaround. My guess would be that installing telepathy-kde-accounts-kcm and kaccounts-providers should solve the issue.
That's not a workaround, it's how is supposed to work. You need at least kaccounts-providers to have a functional KAccounts KCM.