seems more people requested this. As I don;t have the hardware its dangerous to tell you what to do.
You can try to reach out to the author
Or if you want to take a walk on the wild side, I’ll take no ownership or support if it goes wrong or even fail your current installation.
sudo apt -y install build-essential
cd ~
wget https://github.com/Saiyato/volumio-audiophonicsonoff-plugin/raw/master/volumio-audiophonicsonoff-plugin.zip
mkdir ./onoff
miniunzip volumio-audiophonicsonoff-plugin.zip -d ./onoff
cd onoff
npm update (it might be possible that this is not even needed)
npm install --save onoff@6.0.0
volumio plugin install
cd ..
rm onoff
rm volumio-audiophonicsonoff-plugin.zip
and see what happens.