You might be missing key info like a default email address or doesn’t have permission to send mail in the background.
One possible fix to try is open the shortcut in the Shortcuts app and check a few things: make sure your email address is actually set in the “To” field, and that the “Send Email” action is using your Mail app (not a third-party one).
Also, go into Settings > Privacy & Security > Automation and make sure Shortcuts has the right permissions.
I’d break up the shortcut into pieces, and check that each component works as you build out the shortcut. Is there something the shortcut is not working on specifically, like maybe always works for photos, but fails with webpages?