Cannot pair the generic keyboard via automatic PIN

Usually, if you'll wait a moment when the MAC shows up, it'll replace it with the device's name.

At least that's how it happens on my system.

Many keyboards and small devices default to '0000' as their PIN, and the system automatically tries that in many cases. Are you sure the keyboard isn't connected? Try it.

EDIT:

I pulled this from another of my answers concerning getting a BT keyboard to pair before login, it might be helpful for you too. I used it a bunch when the BT pairing system in KDE didn't seem to work properly (never would accept PIN... hmmm, sound familiar?) (a later update finally fixed it, now working properly)

From unix.SE: 'how to get my bluetooth keyboard to be recognized before log-in'

I occasionally use the following script to add bluetooth keyboards to my systems, it adds it at a system level, rather than a user level, which seems to make things work right from the boot, and my keyboard(s) are usable from the login prompt.

As written, you'll need bash (v4.0+ hopefully) and the bluez package, which supplies the bluez-simple-agent, bluez-test-device, bluez-test-input programs.

Most of the code below is to implement a list to allow you to choose which device, it really just boils down to the last 6 (non-comment) lines, if you know your BT MAC Address, you can replace all the choice stuff with a static assignment.

#!/bin/bash
#
# L Nix <lornix@lornix.com>
# setup-bt-kb : allow choosing & pairing a bluetooth keyboard from the console
#
declare -a addrlist
#
while [ 1 ]; do
    echo -n "Scanning for Bluetooth devices ... "
    readarray -n 10 -O 0 -t addrlist < <(hcitool scan|grep -v "^Scanning"|sed -e "s/^[ \t]//g" -e "s/\t/ /g" | head -n 9)
    echo
    echo
    length=${#addrlist[@]}
    a=1
    while [ ${a} -le ${length} ]; do
        echo "$a) ${addrlist[$a-1]}"
        a=$((a + 1))
    done
    echo
    while [ 1 ]; do
        if [ ${length} -gt 0 ]; then
            echo -n "Choose (1-${length}), or "
        fi
        echo -n "'R' to rescan: "
        read -n 1 REPLY
        echo
        case ${REPLY} in
            Q)
                # just quit
                exit 0
                ;;
            [0rR])
                echo
                REPLY=0
                break
                ;;
            [123456789])
                if [ ${REPLY} -le ${length} ]; then
                    echo "Got ${REPLY}"
                    break
                fi
                ;;
            *)
                ;;
        esac
    done
    if [ ${REPLY} -gt 0 ]; then
        break
    fi
done
#
device=${addrlist[${REPLY}-1]}
#
BTADDR=${device/% *}
BTNAME=${device/#??:??:??:??:??:?? }
#
echo "selecting '${BTNAME}' at ${BTADDR}"
#
echo "Pairing with ${BTNAME} (Generally '0000')"
bluez-simple-agent hci0 ${BTADDR}
#
echo "Setting trust level with ${BTNAME}"
bluez-test-device trusted ${BTADDR} yes
#
echo "Connecting to ${BTNAME}"
bluez-test-input connect ${BTADDR}
#
echo "Completed"

I managed to do so via bluez-tools:

sudo apt-get install bluez-tools

List of devices to get the MAC address of my device:

bt-device -l

and successfully connect to it:

bt-audio -c 01:02:03:04:05:06

Keep in mind that the bt-audio feature has been reimplemented probably some time in 2016, so bt-audio is likely no longer available as such.