Tip from https://forum.xda-developers.com/u11/accessories/usb-c-to-3-5-mm-adapter-htc-u11-t3622225/page2 Works for me on htc U11. YMMV.
If you bought a USB-C to 3.5 adapter (without DAC) and get a "Accessory not supported" error message, you can try the following.
Install SoundAbout from https://play.google.com/store/apps/details?id=com.woodslink.android.wiredheadphoneroutingfix
Open App and make sure SoundAbout Service is ON
Select Media audio, choose "Wired Headphones".
Plug in your USB adapter with headphones. Close the "not supported" message
Try playing music on your phone.
Hope this helps!
If you bought a USB-C to 3.5 adapter (without DAC) and get a "Accessory not supported" error message, you can try the following.
Install SoundAbout from https://play.google.com/store/apps/details?id=com.woodslink.android.wiredheadphoneroutingfix
Open App and make sure SoundAbout Service is ON
Select Media audio, choose "Wired Headphones".
Plug in your USB adapter with headphones. Close the "not supported" message
Try playing music on your phone.
Hope this helps!