It might be due to various reasons. Please check if the website that you are using is mcc.nic.in. Candidates who have qualified in NEET 2019 are only eligible to apply for counselling. It is 50% for General Category, 45% for General PWD, 40% for OBC/SC/ST. If you are fulfilling this criteria and are using the correct website, please clear the cache, browsing history, etc and restart your device. Make sure your internet connection is good. If the internet connection is interrupted, the IP address which is being monitored by the Server will change and the session will expire. In such case, change to a different device with good Internet connection and try again.