I don't know what i did wrong or what went wrong but i keep seeing this error messages [Screenshot at 2021-05-16 00-51-16]

Thank you Sir @mdooley . The second link helped. Here the solution is "~$ sudo apt-get update --fix-missing". I just ran it in the terminal and it rectified everything. you are awesome...rather i say you both @24601 & @mdooley are awesome.