Error when I try to create a "Performance Goal" approval rule

Hi,

we would like to configure the "Performance Goal' approval rule. The approver has to be the worker's supervisor (not the Line Manager but another custom supervisor type). We already identified this kind of approver in the Performance document approval rule.
In this case, we selected a Reference 'User' approver type and we insert this formula: GetManager("MBO_REFERENTE_BU",Task.payload.Worker's Assignment.result.requestManager), where MBO_REFERENTE_BU is the Lookup code related to our custom supervisor Type. With this configuration our Performance document approval rule works very well.

I tried to use the same settings in the Performance Goal approval rule, however when I try to save the rule a warning message appears (see the screenshot) which states that "the Task.Payload.Worker's Assignment' is not valid here" (although the settings are exactly the same of the Performance Document Approval rule).