I don't post often, but, your error is so obvious and nobody else has pointed it out…
Your chisel is upside down. It's never going to work at all that way around. It should have the bevel down – I would assume that was obvious as if you were doing this without the extra tool you'd use the chisel bevel down… But I guess proper use of wood chisels is not well understood.
Also you have to be careful to cut inline with the grain, even if the grain is curved, even then you'll get some tearout.
I have an antique stanley version of this tool somewhere, which, from memory (it's not a tool I use anymore and is probably tucked away in my "I have tool problem" storage unit) is a little different, but in principle, the same tool you have. It never worked well, and to be honest was never worth the effort. Careful use of a chisel or appropriate gouge generally worked better.
I now have the veritas version and it is a totally superior tool due to the superior grind on the chisel, but still isn't all that useful, and I wouldn't recommend anyone buy it.
The classic way to fix this before the veritas version was to hollow grind the back of a chisel that you reserve for this specific tool. It doesn't need much, just ensuring that the two edges are the high points. But realistically that still doesn't help much. It's a great idea but in practise, doesn't work.