This error message occurs as where the result of the formula should appear instead it displays "??".
The resolution of this error will be divided into two parts. You will first need to identify the actual detailed error message that is coming from the execution of the formula. And then using this detailed error message you will need to resolve the underlying problem.
You can identify the detailed error message by performing an Advanced Find on the entity named "N52 Trace". This contains all the messages that have been generated by BPA. After you have identified & read the detailed error message you should have enough information to resolve the issue.
Note 1 :
You will need to ensure that at a minimum the logging level is set to "Error" on the "N52 Configuration" entity for BPA to generate trace messages within the "N52 Trace" entity. Please refer to the logging section of this help guide for more detailed information