This is fairly vague..... If you need to use a table you already have open then use the SELECT command, not USE. Think of USE as an "open" command. To actually make the table the current table and to "use" it you need to issue a SELECT command on the table such as:

SELECT myInvoice_rpt

Also verify whether or not you are using a private data session in your data environment, this might not be desirable.

Typically, using the data environment is the easiest way to do things but it is bad form and not scalable. I strongly suggest you look at an n-tiered / Business Object model for data access in the future. You will save yourself a lot of trouble in the long run.