npx wrangler pages dev -- npm run dev

Cloudflare Pages is for static content (HTML, CSS, JS, images and other such files). It will not run a webserver such as Express, Koa or the likes. If you're using those to serve static content then you can just remove that part and use Pages!
If you're doing more dynamic content you can use Functions (https://developers.cloudflare.com/pages/platform/functions). It will run code on request (so still not a webserver in the conventional sense). This would allow for more dynamic content though especially with KV (Key Value storage) and Durable Objects (transactional data storage). Functions are natively integrated with Pages so are super easy to get up and running along your website.
Otherwise, a good old VPS or dedicated server from one of the many hosts out there is for you.
can i add in few files and foldersAdd a few files and folders where? What do you want them to do?
curl --request PATCH \
--url https://api.cloudflare.com/client/v4/accounts/:account_identifier/pages/projects/:project_name \
--header 'Content-Type: application/json' \
--header 'Authorization: <api-token>' \
--data '{
"deployment_configs": {
"production": {
"d1_databases": {
"D1_BINDING": {
"id": "445e2955-951a-43f8-a35b-a4d0c8138f63"
}
}
}
}... (npm install stuff)
20:51:26.907 NPM modules installed
20:51:26.970 Installing Hugo 0.54.0
20:51:27.686 Hugo Static Site Generator v0.54.0-B1A82C61A/extended linux/amd64 BuildDate: 2019-02-01T10:04:38Z
20:51:27.690 Started restoring cached go cache
20:51:27.705 Finished restoring cached go cache
20:51:27.796 go version go1.14.4 linux/amd64
20:51:27.812 go version go1.14.4 linux/amd64
20:51:27.817 Installing missing commands
20:51:27.817 Verify run directory
20:51:27.817 Executing user command: pnpm run build
20:51:27.822 /opt/build/bin/build: line 39: pnpm: command not found
20:51:27.828 Failed: build command exited with code: 127
20:51:28.686 Failed: error occurred while running build command