Google/Amazon changed something : see https://www.widevine.com/news
In future only SD content can be played 
For HD or better we need Google widevine L1 Certificate devices.
Kodi doesn´t have and won´t get certificate
details here : https://github.com/Sandmann79/xbmc/issues/294
Well that’s understandable. It should be the same in Google Chrome or isn’t it?
Even though I have Amazon Prime I rarely use it for video because the Official Amazon App is plain crap. They try to sell their Fire TV devices.
Only way is to use Android but then you can just use their apps too.
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