When DPL encounters something that looks like a function call but doesn't reference one of DPL's built in functions, it looks for a user function library (DLL). Unless you've written such a library, this message means you mistyped the name of a function. The examples below will cause this error.
iif(Start_Year==2018, 0, Cost) Max(0, Profit)
The solution is to correct the name of the function you wanted to call. Remember that DPL is case sensitive (Max is different from max). It is good practice to use the Function button rather than typing in function names by hand. The correct calls are
@if(Start_Year==2018, 0, Cost) max(0, Profit)
Versions: DPL Professional, DPL Enterprise, DPL Portfolio