Your shell is eating your curly brackets, so while you say stash@{1}, git sees stash@1 and that makes no sense to it. Quote the argument (use git stash apply "stash@{1}" or git stash apply stash@"{1}"; quoting either way will work) or reconfigure your shell to only expand curly brackets when there is a comma between them (zsh can be configured either way, bash only expands curly brackets with comma or range between them, other shells may behave one or other way).
