ffmpeg replay input to output giving errors
Try

ffmpeg -i rtmp://xxx.114.173.196/live/ninja -ar 44100 -f flv rtmp://127.0.0.1/live/test


The sample rate cannot be 48 kHz, which is triggering the absurd message that MP3 isn't compatible with FLV although that is its default codec.

This command should work, while avoiding transcoding:

ffmpeg -i rtmp://xxx.114.173.196/live/ninja -c copy -f flv rtmp://127.0.0.1/live/test