The "secret passphrase" is something you decide — something that you will easily remember, but no one else could guess. It is more secure if it is random words (e.g.: "tiger engine dress venezia ocean portal") but could also be a sentence (e.g.: "my crazy cat ate all of my gelato"). It is good to write the phrase you choose in a secure location where it will not be lost.
The error about "Missing Authority" always means that a requested credential was not entered correctly. In this case, you may have accidentally entered your "Posting Key" or "Active Key" into the field where "Master Password" was required. Ensure you are supplying the proper item!
The developer can provide further assistance on Discord: https://discord.gg/ZxFMBr3
Just to be crystal clear: this password is something completely new that I have to invent now, rather than something I've used before? I find the terminology terribly confusing!! I was using my Personal Posting Key or Password - a long one - but I tried the other long ones too....
Yes, the "secret passphrase" is something new, which you will invent now, to secure your info in this new Hive Recovery Account Service.
Is the Master Password the Owner Key?
No, they are separate. The Owner Key and Master Password are different things.