Entering a keyword alone produces an error rather than a search. This is a limit of the archive, not a Subdex restriction.
The rule
Keyword parameters are only honoured alongside an author, a subreddit or a thread ID. There is no supported way to search all of Reddit for a word.
Why it is blocked before sending
An unscoped keyword query fails upstream with a generic error that does not explain the cause. Catching it in the form means you get a message that says what to add, and the archive is spared a request that was going to fail.
What to do
Add a subreddit if you know roughly where the discussion happened, or an author if you are examining one account. Both narrow the search enough for the archive to run it, and both make the query faster.