Everyone why I can’t change SSID in winbox. They said “master interface unknown”
Nobody has a crystal ball … unfortunately.
So it’s hard to tell without knowing what exactly you are trying to do.
From the error message I firstly assumed you are creating a virtual/slave wireless/wifi interface, which always needs to be tied to a master interface.
But that on itself is already strange since whenever you create such a slave interface, a master is always filled in from the available options.
Given your title refers to SSID, it should anyhow be somewhere in the wireless/wifi context.
So I am not really sure where this came from … more info needed on what exactly you are trying to do, how you did it and when the error pops up.