try create .htaccess
file on your root folder then paste this
#remove html file extension-e.g. https://example.com/file.html will become https://example.com/file
RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.html -f
RewriteRule ^(.*)$ $1.html [NC,L]
you can read more about this here https://www.plothost.com/kb/how-to-remove-php-html-extensions-with-htaccess/
EDIT
Since you are using Vercel.com, as per their documentation there's a file config named vercel.json
you can add this
{
"cleanUrls": true
}
the docs said,
When set to true, all HTML files and Serverless Functions will have their extension removed. When visiting a path that ends with the extension, a 308 response will redirect the client to the extensionless path.
for more information pls read their docs here https://vercel.com/docs/configuration#project/clean-urls