You require to install packagekit-qt5 package with their dependencies. Is offered as optional dependency if you install Dolphin outside of KDE Plasma … Most omit to select it. That then will extract/install what you download from Store.
Mind you but OpenAsROOT might not work and we do not recommend to use UI applications as ROOT.
Well, is optional dependency hence will not be marked, and only Pamac UI shows that optional popup AFAIK, and some users that use Dolphin in another DE dislike to add more things to it that could potentially add even more KDE stuff to their installation. Glad was sorted out for you.