Expression rule not returning any value in a column of process report

Expression support in process reports is very limited. AFAIK you cannot use any local variables. BTW, fetching data from DB for each line in report will get you into performance issues.

Not only can you not use local variables, you also can't call any rules or expression rules using keyword syntax for parameters, or it breaks.  I forget whether this applies to the rule-call placed inside the report definition itself, but in the past I've proven that if your rule calls other rules (even very simple ones) using keyword syntax rather than simple positional parameter passing, it won't work at all in the portal report.

Not only can you not use local variables, you also can't call any rules or expression rules using keyword syntax for parameters, or it breaks.  I forget whether this applies to the rule-call placed inside the report definition itself, but in the past I've proven that if your rule calls other rules (even very simple ones) using keyword syntax rather than simple positional parameter passing, it won't work at all in the portal report.