ProfileString is supported as a system function. However it is unsupported in DataWindow expressions. You can read the detailed supported and unsupported information respectively in Appeon for PowerBuilder Help | Functions | System functions | System and environment for functions and in Appeon for PowerBuilder Help | DataWindow | DataWindow operators and expressions.
Attached is a demo for your reference to work around the profilestring function in DataWindow expression. There are two solutions included in the PBLs.
Solution #1: use the Global Function.
Encapsulate the profilestring expression into a global function and then invoke the global function in the DataWindow.
Solution #2: pass information via the DataWindow Retrieve Argument.
Use the system profilestring function to get the necessary information and then pass the information to the DataWindow via the argument of DataWindow Retrieve function.