Transfer music from ubuntu 18.04 to Android device
On Android, pull down your top bar after connecting the phone. You will see a notification "USB charging this device". Tap that and you will see that it defaults to only charging your device. Thus, you can, by default, not mount the drive. Tap "transfer files" on your Android phone to have it mounted in your file manager. To unmount it, you can use the same screen on your Android phone, or use Files.
After this, you can transfer music files, or eventually access it with your music playing software.