That command isn’t valid on Manjaro. At least not by default. Do yourself a favour and learrn to use and understand the package management:
Pamac (Add/Remove Software) is Manjaro’s Package Manager. It is based on libalpm with AUR and Appstream support. It focuses on providing an easy to use interface while still providing a powerful set of features.
That said, xrdp is in the AUR:
$ pamac search xrdp
[...]
xrdp 0.9.22.1-1 AUR
An open source remote desktop protocol (RDP) server
So it can easily be installed using:
pamac build xrdp
Also, please don’t use an older topic for a new question, even if it is your topic originally. Please see:
https://forum.manjaro.org/t/strict-forum-rules-and-guidelines/13680/7
and