Missing environment plasma desktop in Architect installation

You need to clarify which version of the manjaro-architect you have used, the (old) TUI netinstaller 20.0.3 or the GUI-based bundled in all DE-isos of 20.2? If you did the 1st one maybe these will be of help to you

In short it’s advised to use the Manjaro-Architect from the LiveCD/USB (20.2).