Since the mainstream launch of Wayland with GNOME, people have faced problems using xclip, mainly because xclip is designed to work with the X11 clipboard.
Fortunately, xclip has an option to overcome this problem. Just use xclip -selection c instead of xclip. This option emulates the Ctrl + Shift + C action.
For easier use, you can set up an alias by adding the following line to your .bashrc or .zshrc file.
alias xclip='xclip -selection c'