It's an known issue when the Amazon Music HD/UHD is turned on, the multiroom audio for Amazon Music won't work. To work around this, please go to WiiM Home App and select Browse tab > Amazon Music > Settings > Turn off the Amazon Music HD/UHD Beta. It should enable the multiroom or stereo pairing for your Amazon Music.
We're working on enabling the HD/UHD multiroom/stereo pairing for multiroom and stereo pairing now. Please stay tuned!