Check your .htaccess file

Your .htaccess file should have the 2 lines below the index.php line. You can manually add these & it should work also.

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteRule ^join/?$ /wp-login.php?action=register [QSA,L]
RewriteRule ^login/?$ /wp-login.php [QSA,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress