SQL Server Job Powershell is failing
The syntax error you are getting is from the $() this is taken as a token in SQL Server Agent context...so it will always bark at that; removing it should fix the syntax error.

On the copy issue you will likely need to specify the property you want to pass into Copy-Item. Just passing $myfile is not going to work because that contains the full object of the output from gci. So change it to something like:

$myfile = gci $mypath | sort LastWriteTime | select -last 1 -ExpandPropety FullName
Copy-Item -path $myfile -destination $mydestination


Now $myFile will contain the full path to your last file: