Invalid SSL certificate (debug info after installation on Amazon LightSail)

I’ve installed a Lets Encrypt certificate on Amazon LightSail and I’ve followed their instructions (https://lightsail.aws.amazon.com/ls/docs/en_us/articles/amazon-lightsail-using-lets-encrypt-certificates-with-wordpress#complete-the-prerequisites-lets-encrypt-wordpress) to enable it on my WordPress website.

I didn’t get any errors along the way. The final step was to install and enable this plugin. The site is still not loading securely so I enabled the plugin debug mode and it’s coming up with the following

General
Plugin version: 3.3.4
Invalid SSL certificate
SSL is enabled

Options
* Mixed content fixer
* WordPress redirect

Server information
Server: apache
SSL Type: SERVER-HTTPS-ON

Detecting configuration

SSL Configuration
testing htaccess rules…
test page url, enter in browser to check manually: https://www.agentrater.ie/wp-content/plugins/really-simple-ssl/testssl/serverhttpson/ssl-test-page.html
htaccess rules test failed with error: cURL error 60: SSL certificate problem: self signed certificate
checking if .htaccess can or should be edited…
.htaccess does not contain default Really Simple SSL redirect
.htaccess not writable.
converting siteurl and homeurl to https
Mixed content fixer could not be detected: cURL error

Constants
No constants defined

Any ideas what could be wrong?

Many thanks for your help

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