May I ask are you on the Enterprise plan or not?
curl -X PATCH "https://api.cloudflare.com/client/v4/zones/XXX/settings/proxy_read_timeout" \
-H "X-Auth-Email: [email protected]" \
-H "X-Auth-Key: XXXXX" \
-H "Content-Type: application/json" \
--data '{"value":600}'