Hi
The following steps may help when pairing a compatible Bluetooth speaker with your watch (you may wish to check compatibility of the speaker with the manufacturer):
Unpair your speaker from your iPhone or any other device that it may already be paired with.
Restart your Apple Watch:
Check that you are following both the manufacturer's pairing instructions for your speaker and Apple's instructions:
Try moving your watch and speaker to a different location, further away from any other active Bluetooth devices.