Hi, welcome to the forum!
Have you checked the box for MTP devices in Device Preferences? And have you made sure the phone's unlocked and then restarted MusicBee?
If none of that helps, you can try upgrading to the latest version from my signature. For some reason, that's what it took for my roommate's phone (an S7 Edge). My S8 has never had any problems unless I forget to unlock it when I first connect (and I sync to the SD card).