I'd be very curious if anyone else can reproduce this.
I honestly am not sure when this started happening, but it must have been quite recent, as I don't recall having this problem previously.
So, I launch the app, and in the table on the home timeline buffer, I locate a tweet from a user who I'm following, who also is following me. once focused on their tweet, I hit the letter d to compose a direct message to them. I hear Voiceover say, "Send direct message", but nothing further happens. It used to be, it would jump my focus to the text field, and would autofill it with d @UserName, then a space, and be all set for me to type my DM and hit return. Now, nothing is happening past that announcement. I remain in the table, still focused on the tweet, and when I navigate to the text field manually, nothing at all is in it.
All other keystrokes are working as expected such as command+n for a new tweet, return works on a tweet still to reply to it. F still works to favorite things, l is still working to open links in my browser, etc. The only thing seeming to all of a sudden be broken is hitting d for a dm.
I'm totally up to date on the latest version of the app, and have all OS updates as well, running Big Sur 11.3.1.
I'd really be curious if anyone else can reproduce this, and if so, if they know a work around.