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Sometimes there is a conflict between Scratch and the way Windows automatically connects to a Bluetooth device that your computer has connected to before. To get around it, you have to manually tell your operating system to "forget" the device.
To "forget" the device:
Open settings by opening the Windows menu and choosing Settings (the Gear Icon).
Choose "Devices."
In the list of device "Other Devices" click on your device and then click "Remove Device."
That should make the device discoverable by Scratch using Scratch Link.