HTTP Post data from Arduino uno using WiFi
Step 1) Upload software serial from example and send AT, you should get reply as AT OK if your connections are correct.
Step 2) Once your connections are correct and if you are uploading the given code then make sure you put your SSID, password, and Server IP which would be the three Strings: HOST, PATH and PORT.

You could alternatively check your server first by sending it a GET or POST using your browser or another software for testing API's(I personally use POSTMAN).

Proceed further once you perform these checks.