Hi Umais,
Welcome to our community! 
If you get an inexplicable error message, the first thing you could try is to restart Unity. Sometimes, there are just internal problems in Unity, which solve themselves when you close and reopen Unity.
Secondly, if the issue persists, do you use an antivirus program or a firewall that might be blocking Unity? If so, it might be that they either block or delete files.
Did this help?
See also:
- Forum User Guides : How to apply code formatting within your post
- Forum User Guides : How to mark a topic as solved