Windows 10 requires you to enter passkey for a bluetooth mouse. Bluetooth mice don’t have passcodes, let alone means to enter them. You may skip the new interface and go to the old Control Pannel, and add a device from there – it also prompts for a mouse pass-code but has the next button to skip it, however, the mouse will not work. What you need to do is to open devices and remove your greyed out mouse from there. Then reboot the PC, and pair the mouse from the new or the old interface.
Don’t forget that you need to switch the mouse into pairing mode, usually this is done by pushing a small underneath for at least 4 seconds until the mouse light flashes sometimes in different colors. This time the mouse will pair without prompting for a pass-code, and you can finally use it..