I think you got your first line the wrong way around. Currently you're assigning an empty variable (value of it should be
$Env:UserName, thus overwriting the username, not reading it.
I think it should be
$username = [Environment]::Username
or, as noted above, you can access environment variables via the special
$username = $Env:Username
And unrelated to your problem, just a matter of nicer code:
You can put the username directly into the string (which you seem to know, as demonstrated a line above – where you don't need a string in this case, though):
$from = "C:\Users\$username\favourites\*"
You don't need to fetch the user name at all, you can use
which might ultimately be what you're after, here.