Volume API not working

Volumio Information

Volumio Version: 2.0
Hardware: PI Zero
DAC: Khadas Tone Control

I’m new to this community and also as a Volumio user, so forgive me if I don’t know the terminology and context.
I have an IoT system made by myself and I want to include Volumio in the system, through its REST API.
I am able to use all the APIs of my interest, but not the volume API, ie when I try to use it, the answer is “volume Success”, but the result is always NAN (on the GUI); also i can’t use amixer (set) directly on the PI.
From the standard GUI the volume setting is fully functional.
I have read other topics related to audio volume, with suggestions for using specific settings, but in my installation one of the suggested values ​​(Mixer Control Name: Digital) is not available.
Maybe it’s not a Volumio problem, but probably someone can help me.