404 error on new posts

I am very new to WordPress and am not acquainted with coding. My site recently began giving 404 errors on new posts. It does it every time. Unfortunately, I am not always the one it shows it to. I will look and see it and think all is well, and then I will hear from a reader that they got the 404 error notice. If I update the post one or two more times, then as far as I know, the issue goes away. But again, I don’t always get the error, even though others do. Or the error doesn’t show up immediately after I post, but will show up an hour or two later. The post excerpt will show on my Home page fine, but the link to the post may or may not bring it up.

I was told to deactivate all of my plugins. I have fiddled with that some. I have turned off minify. I have resaved my permalink choice numerous times (was told this often corrects the problem). To no avail.

I have found information about this (three years old) on the forum of code to add to the .htaccess file. However, upon looking into that file, there are hundreds of lines of code. How do I know where to place this coding? Does it just go anywhere?

# BEGIN WordPress

<IfModule mod_rewrite.c>
ErrorDocument 404 /index.php?error=404
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

My site is at https://www.robinleehatcher.com

I’m afraid I will break my site completely if I do something wrong, so I don’t want to make a misstep.