You cannot select any formula symbols (display mode)

What causes this issue?

You have attempted to select an operand.
However, you are in display mode, and changes to the formula cannot be
performed here.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

System Response

How to fix this error?

To edit the formula, exit the Formula Builder first, and then call an
appropriate transaction.

Error message extract from SAP system. Copyright SAP SE.