I think it will be much easier to start a new WordPress Multisite instance and migrate your data there. Note you can have both instances running at the same time while you migrate your data so you don't need to turn your website down. We highly recommend you to follow this advice and start a new server.
Apart from the above, we recommend you to create a server backup before doing any other change on it, so you can easily restore to a known state in case anything goes wrong.
I checked your
wp-config.php file, and I found out that you are not properly following the steps in the post you mentioned. The first thing I noticed is that the
define('MULTISITE', true); was not added at the required placed. Can you check it? Note there may be another issues I didn't detect.
I also found the next errors in the Apache log file that you may take a look at
[Mon Jul 27 01:03:13.107502 2020] [autoindex:error] [pid 16636:tid 139911726786304] [client 220.127.116.11:48032] AH01276: Cannot serve directory /opt/bitnami/wordpress/wp-content/plugins/onesignal-free-web-push-notifications/sdk_files/: No matching DirectoryIndex (index.html,index.php) found, and server-generated directory index forbidden by Options directive, referer: http://kuulpeeps.com
[Mon Jul 27 10:25:34.921447 2020] [proxy_fcgi:error] [pid 12525:tid 139911344535296] [client 18.104.22.168:60900] AH01071: Got error 'PHP message: WordPress database error Illegal mix of collations (utf8_general_ci,IMPLICIT) and (utf8mb4_unicode_ci,COERCIBLE) for operation '=' for query SELECT comment_ID FROM wp_comments WHERE comment_post_ID = 225623 AND comment_parent = '0' AND comment_approved != 'trash' AND ( comment_author = '' AND comment_author_email = 'email@example.com' ) AND comment_content = 'Super-Duper website! I am loving it!! Will be back later to read \\r\\nsome more. I am bookmarking your feeds also\\r\\n\\r\\nFeel free to visit my blog post - <a href=\\"http://inbryansk.ru/chat/go.php?url=http://www.psccommunity.org/forums/users/kattiehalcomb39/\\" rel=\\"nofollow ugc\\">\xc5\xf1\xeb\xe8 \xe2\xe0\xf8 \xe1\xf0\xe0\xf3\xe7\xe5\xf0 \xed\xe5 \xef\xee\xe4\xe4\xe5\xf0\xe6\xe8\xe2\xe0\xe5\xf2 \xef\xe5\xf0\xe5\xed\xe0\xef\xf0\xe0\xe2\xeb\xe5\xed\xe8\xe5 \xed\xe0\xe6\xec\xe8\xf2\xe5 \xfd\xf2\xf3 \xf1\xf1\xfb\xeb\xea\xf3</a>' LIMIT 1 made by wp_handle_comment_submission, wp_new_comment, wp_allow_comment', referer: https://kuulpeeps.com/blog/2020/07/04/5-times-wiyaala-made-us-forget-there-was-a-pandemic/