I too am trying to install on AWS Lightsail WordPress. There is a guide from Bitnami here:
About mid-way thru it walks you into installing the LetsEncrypt WP Plugin.
I went though the install, but “activated” the plugin-before the instructions said. By the time I finished it told me “Let’s Encrypt: Registration key is already in use” and I could not move forward.
I tried deleting the directories on the server and starting from scratch, but continue to get the same Error.
Western Spirit Media Services
United States of America