Manage volume from mobile amplification

Greetings, I have volumio 3, and I would like to know, how can I manage the volume of the device, from the mobile app. Thank you

There is a speaker icon in the bottom right corner. Clicking it will open a slider popup (at least in the ios app). To close press it’s bottom ‘X’

True, that speaker icon appears, but when I click on it, no pop-up window opens, but a forbidden symbol appears. Thank you

Have you a volume mixer set in Volumio playback settings?

no, in the volume options, in mixer type “none”, you should change the option to “Software”. Thank you