“KDE’s calendar app”: I didn’t give you the name, I just know KDE have a few, and they make really good apps.
[ion@ion-desk ~]$ sudo pacman -S kdecalendar
This will not work, because you are giving it a direct command to an app that doesn’t exist by that name, and pacman will not just run a search, interpret, correct, or change erroneous entries… for you, you need to use the right syntax for it to do a search, which I don’t know because I don’t use pacman, but I do use yay. If I type: yay appname it will present a numbered list of all it finds, which will be of differing versions and installers, but a general search for yay calendar will give you a very long list of anything with the word in the repos, which will be very hard to get through. At the bottom you can type in the number/s of your choices and hit [enter] and the install routine commences.
I found it much easier, and much more informative to just look online for software, read up on the apps you find to get better information to narrow down your selection, get the apps lower case terminal designation that will work, then install the ones that sound usable for you, try them, uninstall the ones you don’t, and keep the one you do like.
If you prefer GUI apps, or have difficulties with the command line which you seam to, just like me, then there is Pamac, Manjaro’s GUI for pacman which has a search function, but it would work better with “calendar”, then “calandar kde” rather than “kde calendar” (may or may not be case sensitive). It would be a very good app if it weren’t so flaky as to breaking often, having so many separate versions for package format combos…, rather than just one installer with package formats as choices on install or in the installed app where it could just install the dependencies for the ones you chose on choosing. It also breaks often, but is great when everything works.