It seems like you’re having a lot of issues – it really shouldn’t be working that way. Here’s how I do it on desktop; should work on the Deck.
Do a pacman -Syu as others have suggested before installing anything new.
From terminal (X used for AUR package name):
Install auracle with pacman -S auracle; it’s a utility to search and install from AUR easily.
auracle clone X will clone the project in the working directory.
auracle buildorder X will give you a list of dependencies needed to build it. If things say satisfied, you already have them.
install the dependencies with pacman -S. You can get fancy by echoing them to a file and using something like sed, or you can just type them manually.
cd into the directory you cloned and build the package with makepkg.
once done, hit up arrow until you see your installation line for those dependencies with pacman. Change that -S to a -Rs (remove them plus any dependencies installed with them).
install the package you just built with pacman -U name-of-package.zst.
profit
If this doesn’t work for you, check where it doesn’t work. It sounds like you may have an issue with pacman rather than the AUR.