Ubuntu 18.04 cannot connect Bluetooth to Android phone

There are a number of reasons why you may not be able to connect to a Bluetooth device, such as a phone or headset.

Connection blocked or untrusted

Some Bluetooth devices block connections by default, or require you to change a setting to allow connections to be made. Make sure that your device is set up to allow connections.

Bluetooth hardware not recognized

Your Bluetooth adapter or dongle may not have been recognized by the computer. This could be because drivers for the adapter are not installed. Some Bluetooth adapters are not supported on Linux, so you may not be able to get the right drivers for them. In this case, you will probably have to get a different Bluetooth adapter.

Adapter is not switched on

Make sure that your Bluetooth adapter is switched on. Open the Bluetooth panel and check that it is not disabled.

Device Bluetooth connection switched off

Check that Bluetooth is turned on on the device you are trying to connect to, and that it is discoverable or visible. For example, if you are trying to connect to a phone, make sure that it is not in airplane mode.

No Bluetooth adapter in your computer

Many computers do not have Bluetooth adapters. You can buy an adapter if you want to use Bluetooth.

Also Check the link below

https://help.ubuntu.com/stable/ubuntu-help/bluetooth-connect-device.html.en