I am having difficulty adding an account in “Create new account.” When I type Reddcoin or RDD nothing shows up in the Altcoin box. When I enter a Reddcoin address it says it is an invalid LTC address. I cannot add a new RDD account.
It appears that Reddcoin has unfortunately up to now never been traded in Bisq.
According to the delisting rules, Reddcoin has been delisted.
Which explains your issue.
RDD still appears in the displayed altcoins list if and only if you have added before.
But normally it should no more appear (it’s some UI issue).