Are you still able to use Acrobat despite the warning? If not, you may be out of luck. Since XI is so old, it probably does not recognize your current browser as being compatible because it has never seen it before and knows nothing about it.  e.g. Edge came out after Acrobat XI so is unknown to it.  Acrobat used Internet Explorer as the required browser for signing in to Acrobat products on Windows in those days, and since IE 11 (the latest) is now deprecated and disabled on Win 10 in favour of Edge, you have no way to sign in now.

Acrobat XI supported the following browsers on it's last update (11.0.23 Nov 2017): Internet Explorer 11; Firefox ESR (of that era); and Chrome (of that era).

Acrobat XI is an old and unsupported version, and is no longer compatible with any current OS, so problems are to be expected. To increase the chances of it working properly make sure you have all updates installed (up to 11.0.23) and run it in Compatibility Mode with an earlier OS, like Windows 7.

Updates are available here:
ftp://ftp.adobe.com/pub/adobe/acrobat/win/11.x/

You must use a dedicated FTP client to open the link, it won't work in most browsers.