Which error is this?

[Paleophyte]

Which formal structural error is this? Is it still considered Affirming the Consequent if they’re using negations of p and q in the second premise?

It's an unfounded premise.

1) If Thor does not exist then thunder does not exist. <--- Why assume that this is true?
2) Thunder exists.
3) Thor must exist.

That's entirely sound and a decent example of denying the consequent. If Thor is the only possible cause of thunder then the existence of thunder logically requires the existence of Thor.

But it depends entirely on the validity of P1. The instant that you introduce the possibility that thunder can exist without Thor then denying the consequent fails and the argument collapses.

1) If Thor does not exist then thunder might be caused by something else.
2) Thunder exists.
3) Thunder has an indeterminate cause and we can't say a bloody thing about Thor's involvement.

It's a lovely example of an argument that's logically sound but entirely unfounded. Which looks to have been the point of the person posting the obviously ridiculous Thor exists argument.

It's argument by assertion. Thor is the only possible source of thunder. God is the only possible source of morality. And that's arse backwards since they have to prove that god exists first before assigning him attributes.