My phone got locked, its showing "IPHONE UNAVAILABLE" on the display screen

First of all, put your unavailable iPhone into recovery mode after connecting it to your computer with a USB cable. Then the Finder or iTunes will show you a pop-up screen with the Restore option.

If you don’t know how to go to recovery mode, click the link recovery mode to refer to the instructions, after connecting your iPhone to the computer.

Secondly, click Restore iPhone.

Thirdly, set up your iPhone after the restoring process.

It would be better if you can please re-post with a screenshot, in order to understand the phenomenon better. Images/pictures speak louder than words. 

Please understand this is a public forum: While attaching a Screenshot, do not provide any personal credentials, such as IP address, Card detail, email id, apple id, IMEI number, Serial Number, Phone number, Order ID, if you are sharing a map ensure your location is not identifiable, etc.

Have you forgotten your passcode?, See this Apple article: If you forgot your iPhone passcode

https://support.apple.com/en-us/HT204306

Sravankra this is the new error message for wrongly entered passcode since about 15.2

The Erase Iphone at bottom right means it can be wiped without use of a computer.