field symbol has not yet been assigned, ALV

The error is this:

Field symbol has not yet been assigned.

Error in the ABAP Application Program

The current ABAP program "SAPLSLVC" had to be terminated because it has

come across a statement that unfortunately cannot be executed.

When I want to display the data with alav grid, I could not achieve it and got the error described above.

What is the cause of this problem?

Thanks.