Dolphin root does not work if you downloaded it from the AUR using pamac.

[Glisix]

Installing kde-servicemenus-rootactions on the AUR does not work. I found an alternative workaround.

Install yay. You can refer to this link to install yay

Once yay is installed. Open a terminal and enter the command
yay dolphin root
It will show 4 options.

Choose the 4th option to open Dolphin via Polkit. All the other options is to open dolphin via ksudo.

Once installed. Open Dolphin. Rightclick anywhere in a blank space. And select root actions, and click Open in File Manager.

It will prompt you for your password and dolphin is now in a plain white background with root access.