DIAGNOSIS The Dot's Bluetooth is in the wrong mode. It needs to be in pairing/discoverable mode, not already connected to a phone.
FIX Say, "Alexa, disconnect my Bluetooth." Then say, "Alexa, pair Bluetooth." The light ring will pulse blue. On your laptop, search for new devices and select "Echo Dot-XXX." Confirm the pairing on both devices.
IF THAT DIDN'T WORK Clear the Dot's Bluetooth memory. In the Alexa app, go to Device Settings > your Echo > Bluetooth Devices and clear all paired devices. Restart the Dot and try pairing again.
HEADS UP When paired via Bluetooth, the Dot's microphone is disabled for the connected device. You cannot use Alexa voice commands to control the laptop audio.