ITunes For Mac Error. Please Help

When iTunes for Mac is started, this error message appears.
iTunes For Mac ‘ERROR MESSAGE’
‘The required file was not found or has a permissions error. Correct this permissions problem and try again, or deauthorize this computer if the permissions cannot be changed.’
I am very confused and can’t wait to download some music and video from iTunes soon. Could anyone help to fix this error?
Thanks in advance.
