Make sure that your Echo/ Echo Dot were connected to a Wi-Fi network, please refer to the below link to setup your Echo/ Echo Dot.
Step1: Go to https://alexa.amazon.com from Safari, Chrome, Firefox, Microsoft Edge, or Internet Explorer (10 or higher) on your Wi-Fi enabled computer, then select Settings and click your Echo/ Echo Dot’s name.
Step2: Click Bluetooth.
Step3: Click‘Pair a new Device’.
Step 4: Fine Bluetooth icon and open ‘Display classic View’ , then your Echo/ Echo Dot should be appear as a new icon.
Step5: Double click the Echo/ Echo Dot’s icon to search device, a number of available services should appear in the bottom left hand corner.
Step6: Right click the Echo/ Echo Dot’s icon and select ‘Listen to music’, it will have a voice prompt once connected successfully.
Plug & Play
Step 4: Go to the settings on your PC, turn on Bluetooth.
Step5: Click ‘Add Bluetooth or other device’.
Step6: Click your Echo/ Echo Dot’s icon.
Noticed: Your PC’s name will shows on the setting page if your Echo/ Echo Dot connected successfully with the Bluetooth Dongle.