Site without prefix(www.) keep redirect to the signup page.

You could try an .htaccess redirect though define( 'NOBLOGREDIRECT', 'https://yourdomain.com/' ); says ‘If there’s no blog, go to https://yourdomain.com/’ and it SOUNDS like you mean ‘If there’s no blog, go to https://www.yourdomain.com/’

So you shoulda used define( 'NOBLOGREDIRECT', 'https://www.yourdomain.com/' );

htaccess will look something like :

RewriteEngine on
RewriteCond %{HTTP_HOST} ^your_domain.com$
RewriteRule ^(.*)$ https://www.your_domain.com/$1 [R=301]

Put it on the top of your .htaccess, well above the WP section.

I tried, it’s not working ??
showed up this:

Moved Permanently

The document has moved here.

Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.

oh I tried the define( 'NOBLOGREDIRECT', 'https://www.yourdomain.com/' );

when I did that, it works but my 404 page doesn’t work. so I removed that back. I want both to work.

when I did that, it works but my 404 page doesn’t work.

What theme?

got it! got it! thanks ipstenu! I cleared the cache and it works with the code that you gave(htaccess)..