E: Package 'steamcmd' has no installation candidate

Solution 1:

A quick scan (rmadison steamcmd)

guiverc@d960-ubu2:/home/wallpapers$   rmadison steamcmd
 steamcmd | 0~20130205-1 | xenial/multiverse  | source, i386
 steamcmd | 0~20130205-1 | bionic/multiverse  | source, i386
 steamcmd | 0~20180105-3 | focal/multiverse   | source, i386
 steamcmd | 0~20180105-4 | groovy/multiverse  | source, i386
 steamcmd | 0~20180105-4 | hirsute/multiverse | source, i386

(or via web browser; https://packages.ubuntu.com/search?suite=all&section=all&arch=any&keywords=steamcmd&searchon=names)

shows the command is only available for i386 architecture; ie. not a raspberry pi using armv7 or arm64. Your pi has the wrong CPU for steamcmd