'MeasureGet(RT: 18 - Measure not defined for measurepoint)'
This error can occur if the MeasureGet function is configured to try and get measure values of a class which do not exist, for instance the server busy time of a Transaction (rather than a Page Time).
As an example, MeasureGet("TMain", MEASURE_PAGE_SERVERBUSY, MEASURE_KIND_LAST, fValue); references the transaction 'TMain' instead of an individual page timer. In order to get values from a transaction such as TMain, it is necessary to use a parameter such as MEASURE_TRANS_TRANSOK.
Please check the SilkPerformer help section "MeasureGet Function" for more information on the objects, types of measure and measures values which can be returned.