Hey, for Godot web exports on itch.io, clipboard access is blocked because the game runs inside an iframe. Because of that you might have to use a popup that displays the save string so players can manually copy and paste it.
This reddit post might help you set up a similar workaround:
https://www.reddit.com/r/godot/comments/1f5ycz6/clipboard_copypaste_workaround_for_webhtml5_build/