Notebook Wireless and Networking

I'm trying to connect and use an Apple Wireless Keyboard. It's a bluetooth keyboard. It was paired and working fine with this Envy notebook until yesterday when the batteries got too low. I put in new batteries and have not been able to pair since.

The Windows 10 connect process (right click bluetooth icon in tray > Add a Bluetooth Device) immediately displays the keyboard in its list of available bluetooth devices. I click it and the " Connecting..." process starts. I wait a while and get the message  "Check the PIN and try connecting again."  I understand that normally when connecting a bluetooth keyboard, a PIN pops up and one must type it on the bluetooth keyboard to complete the process. But no PIN ever appears, so there's nothing for me to type, and I have to cancel the process. Here's what I've done so far to troubleshoot.

• I've tried the process over and over again.
• I moved the keyboard very near to, or several feet away from, the laptop.
• I moved the notebook and the bluetooth keyboard to other locations in house, further away from other wireless devices.
• I turned off all the other bluetooth devices in house.
• In Device Manager I deleted everything under the Bluetooth category, selecting to delete the driver files on the computer, and rebooting. (After reboot, everything comes back by itself.)
• The HP support assistant offers no updates.
• Windows Update offered a more recent bluetooth driver. I installed it and rebooted.
• I took the bluetooth keyboard to another Windows PC. It paired to that PC without issue. The PIN appeared, I typed it on the keyboard, and done.
• I took the keyboard from that PC (also an Apple Wireless Keyboard) to this Envy notebook, and have the same problem.

Nothing helped.