Error GET https://domain.com/wp-json/contact-form-7/v1/contact-forms/3/fill/ 404 POST https://domain.com/wp-json/contact-form-7/v1/contact-forms/3/feedback/ 404
another reasons can be:
# Add trailing slash to the end of the link
RewriteEngine on
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !(.*)/$
RewriteCond %{REQUEST_URI} !^/wp-json
RewriteCond %{REQUEST_FILENAME} !\.(gif|jpg|png|jpeg|css|js|xml)$ [NC]
RewriteRule ^(.*)$ https://%{HTTP_HOST}/$1/ [L,R=301]
# End Adding trailing slash to the end of the link