In the lower left of the "Edit password" Dialog there are Four Icons. Select the Rightmost Icon which looks like a Wrench.
The "Edit Form Fields" Dialog has Field name on the Left and the Value to paste into that Field.
I have seen that the Field name has changed (on the website) and I have deleted the Fields.
This seems to happen on some sites which have more than One login screen you can use and LastPass saves the different Fields in the "Edit Form Fields" Dialog so it can select the matching Value to use in that Field on the website. This is where Fill-in data is stored such as a website that requires a PIN: number to be entered during the login.
Then you have to select the "..." Icon in your browser and Click on: "Account Options", "Advanced", "Clear Local Cache"
Then you have to select the "..." Icon in your browser and Click on: "Account Options", "Advanced", "Refresh Sites"
This may or may not help...
As a last resort:
Click the "..." Icon and click "Add Item"
Click the "..." Icon and click "password"
Delete all the fields and enter into the
URL: field http://www.amazon.com
Username: (your username)
Password: (your password)
Click Save button and try it out.
This will eliminate trying to reuse the Password Item you already have which might remember something which is causing the issues you are seeing.