Formula cannot be saved because it is incomplete

What causes this issue?

You have attempted, either through explicit save or by exiting the
Formula Builder, to save a formula that is not complete and not without
syntax errors.

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?

Correct the formula so that the status light is "Green" and save the
formula once again. Or exit the Formula Builder using the function
"Cancel", without saving the changes.

Error message extract from SAP system. Copyright SAP SE.