Hi I accidentally deleted the red mark, how can I make it appear again?
Dorico calls that a “signpost”. In this case, it indicates what the key signature is. To put it back, you can either use the key signature popover (Shift+K, C, Enter) or the key signature panel.
Thank you. Does it only appear when it’s in the key of C? It doesn’t appear when it’s in the key of D.
It appears whenever you’ve specified a key but there’s no key signature to display. In your screenshot, you’re in the key of C major, and the piano doesn’t display a key signature for that key.
If you add a horn in D, set the key to D major, and view the horn part in transposed pitch, you’ll see a D Major signpost.
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I see^^ Thank you
A minor, too! 
Signposts show you something that can’t otherwise be seen in the score. This includes System/Frame Breaks, hidden objects like time signatures or tempo indications, Note Spacing Changes, etc, etc.
Note that “No key signature” and “C major” are slightly different things, which is why it’s important to know which one you’re using.