No Powershell, aspas simples '
indicam que a expansão da variável não deve acontecer dentro da string. Você deve tentar com as aspas duplas "
:
Get-QADComputer -NotLoggedOnFor 90 -SearchRoot "doman.name/OU1/$OU2"
Nos documentos:
PS C:\> Get-Help about_Quoting
...
SINGLE AND DOUBLE-QUOTED STRINGS
When you enclose a string in double quotation marks (a double-quoted
string), variable names that are preceded by a dollar sign ($) are
replaced with the variable's value before the string is passed to the
command for processing.
...
When you enclose a string in single-quotation marks (a single-quoted
string), the string is passed to the command exactly as you type it.
No substitution is performed.