Digital Music PlayerNWZ-M504

You cannot connect via the Bluetooth function.

Check that the Bluetooth function of your Walkman and smartphone are turned on.
Check that the model name of your Walkman is selected in the Bluetooth connection screen of the smartphone. If not, tap the model name of the Walkman.
Your Walkman cannot connect with a smartphone that is not paired. Check that the Walkman has been paired with the smartphone.
The pairing information might have been deleted. Make the pairing operation again.
If your Walkman and the smartphone are too far from each other, a Bluetooth connection may not be established or may be interrupted. Place the Walkman and the smartphone closer.