Unidentifiable substring 'iif' in expression. The parser reported error message: Unrecognized identifier

Hi Community, 

does anybody ever faced this error: 

Severity Code Description Project File Line Suppression State
Error Path: [AxReport/ABCFreeTextInvoice1/Designs/Report/Expression]:Report: Unidentifiable substring 'iif' in expression. The parser reported error message: Unrecognized identifier. Report_experiment (CUS) [ABC_ApplicationSuiteExtension] C:\AOSService\PackagesLocalDirectory\ABC_ApplicationSuiteExtension\ABC_ApplicationSuiteExtension\AxReportABCFreeTextInvoice1.xml 0

I have added a new textBox and put in this expression: =First(Fields!ABCPeriodOfPerformance.Value, "FreeTextInvoiceDS")

Any recommendations on how to solve this error ?