This won't solve not having a Master, but try the code 19751 or 66098 when it asks for one on the screen to change the programming. I had the same issue and the first code worked for me.
Eventually, I went to the dealer where I bought mine and told them I didn't think it was right they sold me a truck with only one key - even though it was my fault for not insisting on one and they offered me a new key and programming at their "cost".
It is surprising to me that they cannot look up the code by VIN #, but they can't. I think there are a few codes out there that may be their default programming code. They gave me 2 to try and luckily one of them worked.
My truck is a '19 so they may have different codes for different years.
Buying a new fob and finding someone with Forscan is the other thing you can do. Most of the things I found online could only duplicate the fob you have, but not create a Master.
I'm sure 1ags will have a better answer.