Thanks for the additional details. I wouldn't expect that to work as that isn't one of the modules available through the cPanel list.
If I perform that command on my personal machine I see this in the output of the installation:
In file included from /root/tmp/pear/apcu_bc/php_apc.c:35:0:
/opt/cpanel/ea-php80/root/usr/include/php/ext/apcu/apc_arginfo.h:4:3: error: #error Not supported on PHP >= 8.0
# error Not supported on PHP >= 8.0
so it seems there are some dependencies that don't yet work well with PHP 8.