Which error is this?

[Rev. Rye]

It took me trying to check it against multiple formal fallacies for half an hour to figure out that the problem was less the form than the initial premise. I can remember taking some philosophy class and hearing the teacher talk about how sometimes, people can construct arguments that seem airtight until you realise "oh, wait, the initial premise actually doesn't work." I think this was during a unit on Kant, and I think that the OP's argument was an example in miniature.