If you get the error “An xpath expression must be of the form …” (in an expression shape of a BizTalk orchestration), here’s one possible solution.
I was using a multi-part message, so instead of msgCreateSPServiceOrderAckResp, I had had to specify msgCreateSPServiceOrderAckResp.parameters.
The compiler just shows the error “The expression that you have entered is not valid”. But when you find the small red mark, and mouse over it, you see the following:
Here is the corrected version: