The Awazon captcha is six characters and case sensitive, and most rejections come down to case or a stale page.
Check the case
Read the characters carefully and match the case exactly. A capital typed as lower-case, or the reverse, fails the check. This is the most common cause.
Reload for a fresh puzzle
If the page has been open a while, reload it for a fresh captcha and fresh field names before trying again. A stale page can carry an expired challenge.
Type into the real field
The login has decoy fields whose names rotate. Type into the visible real field only. If a password manager is filling the wrong field, fill the visible field by hand instead.
Confirm you are on the real site
Before troubleshooting further, confirm the captcha's printed address matches your bar. A clone's captcha can behave oddly because it is not the real one.