In trying to login to the “Send To Kindle” application on Windows I found that my valid username and password did not work. I contacted support and, although helpful, they could not help resolve the problem. I then remembered that I had 2FA enabled on my Amazon account. I disabled 2FA and successfully signed into the screen below. I then went back and re-enabled 2FA on my Amazon account.
After disabling 2FA I could sign into the app.