you cannot use the AD attribute DisplayName as the value for the paramter -Identity. You have to use the parameter -Filter then. Assumed your header in your CSV file is named “DisplayName” you could do it like this:
… untested because I don’t have an AD to test at the moment.
If you want to use the parameter -Identity what might be a little more precise because it uses unique identifiers you have to use one of the following AD attributes:
A distinguished name
A GUID (objectGUID)
A security identifier (objectSid)
A SAM account name (sAMAccountName)
Edit:
… and I almost forgot - you should not use $PSItem for own variables as it is a reserved variable from PowerShell. It is equivalent to $_.
Edit #2:
… Doug is right. Changed the code above for future visitors.