This is due to the voting option.
You have enabled voting on your collection and possibly set the voting date to start now.
Since the whole collection closes when voting opens, this is the reason that you are not able to add
Voting works by allowing users to enter links (start time until voting opens), then it closes the collection allowing people to
vote on the links and then it completely disables both voting and adding links. So, the times should be set somewhat like this:
Open time: now. People are able to submit their links
Voting starts: in a couple of days (People stop submitting links, they start to vote on the existing ones)
End time: People see the results of the voting. No more submissions or voting is allowed
You can easily change the times from your dashboard by clicking on edit collection.