When you run Current Data Table() with no argument, it returns a reference to the current table. When called with an argument, it should be a reference to an open data table. Here is another example that will give the error in question, on the second call to Current Data Table(dt1):
dt1 = Open("$SAMPLE_DATA/Big Class.jmp");
Show(Type(dt1)); // "Table"
Show(IsEmpty(dt1)); // 0
Current Data Table(dt1); // ok
Close(dt1);
Show(Type(dt1)); // "Table"
Show(IsEmpty(dt1)); // 1
Current Data Table(dt1); // gives error
As mentioned by @Georg above, after running the script you can hover over variables in the script to get some information, or use the embedded log to query the values. If you highlight the dt1 variable and evaluate (run) the script, the log will display:
Data Table( "UnknownTable" )
In the example above, I also use IsEmpty(dt1) to show one way that you can detect with JSL that the variable refers to an object that no longer exists.