Can't Join A Aternos Server For A Unknown Reason

This issue is happening on 21w05b. The affects versions won't let me put it in.

 (Note that this is not really a bug, but I can't find specific info on this multiplayer issue anywhere else.)

This multiplayer issue has appeared for me recently. I do not know where else to post this, and looking online, I cant find a similar multiplayer issue like this. My versions for the server are exactly the same as the aternos one. Does anyone know what this multiplayer message means?