The error message is actually pretty clear. The "pattern" you provide for your Select-String is empty. Why? Because in Powershell variables start with a $ sign. So Powershell "thinks" you want to provide the two variables "$11t" and "$dpo3". And they are – I assume – empty because you did not declare them, right?
What do you actually mean with "$11t$dpo3"? Should it be taken literal? Then you could enclose it in single quotes and use the parameter -SimpleMatch. If it should be a regular expression I think it does not make sense this way. Because the $ sign represents the end of the line or the end of the string and it does not make sense to have two of them in one short string.