Bluetoothctl connects and disconnects repeatedly

try these steps;

• put the keyboard into pair mode (k380 you would hold the bt key you want to assign the device too)
• at pi console, run bluetoothctl
• type power on
• type agent on
• type pair 0C:4D:12:11:01:E4
• type the pair code into the keyboard as prompted
• type connect 0C:4D:12:11:01:E4
• type trust 0C:4D:12:11:01:E4
• type agent off
• type quit

hopefully you should have a bt keyboard connected. you may have to do additional work to ensure the BT will power on and pair on restart of the device.

you require to enable the agent so that keyboard can do the pair code thingy. some keyboards may vary.

hope this helps.