I could not access my website wp-admin after installing SSL. And error with mixed content

I’d put the HTTPS redirect at the top of the htaccess file, not after the WordPress elements. See this page in the Codex for htaccess suggestions: https://codex.wordpress.org/htaccess . You need to redirect to https first before doing the WP rules in htaccess.

You may also need to change any http to https in your database. I use a plugin like “Better Search and Replace” to do that, there are other similar options.

And make sure that you change the URL in the wp-option table to https (in two places).