Diagnosis
The speaker is already paired to the maximum of 8 devices and its memory is full, or your laptop's Bluetooth driver is outdated.
How to Fix It
Clear the speaker's pairing list. Turn the speaker on. Press and hold the Bluetooth button for about 10 seconds until you hear "Bluetooth device list cleared." Now put it in pairing mode (blue blinking light). On your laptop, delete old Bose Bluetooth entries from its list before trying to add the Flex anew.
If That Didn't Work
Update your laptop's Bluetooth driver from the manufacturer's website. Ensure your laptop supports Bluetooth 4.2 or higher. Some Windows PCs have weak Bluetooth radios. Try moving the laptop closer to the speaker during pairing.
Heads Up
The Flex can only actively connect to one source at a time. If it's connected to your phone, it will not appear in your laptop's discovery list until you disconnect from the phone.