Taxes should be submitted via the creator dashboard instead of through the separate Amazon Payment Portal, which has a separate login that may be different than the users Amazon account.

Twitch should also partner with a tax chain (H&R Block, Jackson Hewitt, etc.) and have the partner offer assistance to streamers who need assistance filing taxes with the royalties paperwork, as it's not straight forward and you can't really find clear, concise information of how to file these online.