I have just installed Windows 11 Pro and was not able to reproduce the issue you describe.

With mIRC v7.67, if I join a channel and then double-click on a nickname, either in the channel nick list or a nickname in the window, it opens a query window with an editbox at the bottom, as expected.

Are you using any scripts/addons/DLLs with mIRC?

Can you please type the following in your status window and report the result?

//echo -a $os $version $iif($beta,04 $+ $beta $+ ) $md5($mircexe,2) $file($mircexe).sig $alias(0) $script(0) $dll(0) $com(0)