Shuli,

In my experience, this error means that you are are trying to access a table that is not open. Somewhere you have closed the table that you are trying to SELECT. You might have a USE command following a SELECT myTable that is closing your table. Or you might have a USE IN myTable command that is closing your table.

You could put in front of the command where the error is happening a set of commands such as -
IF !USED('myTable')
USE myTable
ENDIF

That would prevent the error from happening, but it is better to find where the table is being closed so that it can be SELECTed without error.

Good luck with it.