Externally executed command error on Calculator Variables

Hi,
I'm trying to create TSS variables in a Bio_P removal tank, three anoxic and three aerobic tanks, but I keep getting these error messages:
System error: Externally executed command has returned error code 1
I/O error: Cannot remove file: C:/Users/s131502/Desktop/Project2/Project2.Dynamic.CalcVar.TSS_Aer_tank_3.dll

What do they mean?

What kind of variable do you need? I assume it's a plain algebraic expression - with no articulated statement, right?
If so, it is most likely that you have typed it in incorrectly. Remember:
[list]
[li]the code is case-sensitive (e.g. if a block is called "ASU_1", the string ".asu_1.Vol" is going to fire that error message)
[/li][li]the syntax is: .[NameOfBlock].[NameOfQuantity] (remark the 'dot' in front of the block name and in front of the quantity name)[/li]
[/list]

Thank you very much, Enrico. But what if the quantity I'm using is a calculator variable? What do I type in the  [NameofBlock] section?

Then it is ".[NameOfQuantity]" only, as a Calculator Variable is some kind of top-level object