Dear @josh,
I could reproduce the above error with an invalid passphrase. (Simply entered ‘soon’ as the 24th word)
There are two cases: (1) either the last word is really incorrect. In that case, simply OMIT the last word ( and the space before it), give your addrress in the first box, and run the procedure.
(2) the 24 word mnemonic is not a standard Algorand wallet mnemonic at all. Try to remember the type of the wallet. See also this theme.
For better security, the HTML/JS procedure will be published on my Github soon.