You cannot select any formula symbols (display mode)

What causes this issue?

You tried to select an operand.
However, you are still in display mode. You cannot make changes to the
formula in this mode.

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, first leave the Formula Builder and then call up a
transaction for making changes.

Error message extract from SAP system. Copyright SAP SE.