ModelSim SystemVerilog compile error

You need to use output or inout instead of ref arguments. Passing a ref argument requires a whole variable, not a part select, which is considered an expression.

About the only time you need to use ref arguments is when you have a time-consuming task and you need updated argument values as the task call progresses.

Since this task call is from an always_comb block, opTF really should be a void function. A function call has a guarantee that it will not consume time (has no blocking delays).