This message usually indicates that your wireless driver does not support multiple virtual interfaces or the specific combination of virtual interface types (e.g. ap + sta).
The objective of creating this interface is to convert it into an AP with hostapd that is supported according to the hardware capabilities (up to 16 AP's). Do you know how can I fix this?
Best is to let hostapd deal with this, it would destroy and recreate the vif anyway to put it into access point mode.
There is a hidden type value to iw, called __ap which will create an interface suitable for ap mode: iw phy phy1 interface add testing type __ap
According to the interface combination list, your driver supports either 16 concurrent AP interfaces or one sole managed interface. It neither supports multiple managed ones, nor a managed one in conjunction with existing AP ones.
Some drivers will also spawn a wlan0 default managed interface on boot, you might need to destroy that first before creating further vifs.
As I wrote, maybe there's a preexisting wlan0 / wlan1 interface which you need to remove first. Best is to purge all interfaces on phy1 before trying to setup a new one.
Try setting up another network through /etc/config/wireless first and see if that works. If it does, your command sequence is likely wrong and you need to do further steps. If it does not work, the driver capabilities might be wrong and it isn't actually able to run multiple AP mode interfaces at once.
It is not working through /etc/config/wireless. The thing is: I can add a third SSID on web GUI, but I want to do it from the command line and using hostapd. It is supposed to work.