I was strugling with the same issue.. and found out someone who know how to be able to "see" the PIN code.... using command line....
It'S now working for me .. i have Ubuuntu Mate on a Raspberry Pi4 - 8 GB
1 - Open Terminal
2 - install bluetoothctl if not there (but in my Ubuuntu mate 20.04... it was there)
3 - bluetoothctl
then # start to appear
agent on
scan on *** note the MAC adress of the keyboard. You will need to activate the bluetooth pairing on the keyboard *** let's assume the MAC of the keyboard is : ab:cd:ef:12:34:56 .. Stop the bluetooth pairing on the keyboard otherwise, the following option wont work
scan off
trust ab:cd:ef:12:34:56
pair ab:cd:ef:12:34:56
** start bluetooth pairing **
when pin appear, type it on the numeric keypad of the keyboard and hit "enter".
enjoy !