Hi Om Verma, my apologies for the wait to recieve support, it’s been a hectic time at my school, as I’m also a teacher on top of maintaining Gibbon 
I’ve had a look at your screenshot, which is helpful to identify the issue, thanks for sharing it. The gibbonPersonID field is a very special database ID used by the system. It looks like you’ve provided a username for this field, rather than a valid gibbonPersonID, which is why Gibbon is giving and error. Gibbon can accept usernames, but it depends on the setup of the import time. If you can share your YML file, I can likely suggest changes to help make this work.
It also looks like Gibbon is attempting to do an INSERT action which is likely not your intent to create new users. Be sure to select Update Only from the import page when configuring your import.