It is possible that your phone or tablet is already paired with something else. Maybe you had your phone paired to a car stereo or a speaker in another room of your house.
Reset the Bluetooth on your phone (or iPad, tablet, etc) in Bluetooth settings.
– Turn Bluetooth OFF and then ON again right from your device (i.e. phone).
– “Forget” your Bluetooth device and redo pairing.
This will reset any existing Bluetooth connections, allowing you to reconnect to your speaker. You may also need to do the pairing process again.