How to enable the logical function section in the line rule or succeeding and preceding rules?

The logical function will only be enabled when you use Activity datafields, where you test multiple instances of the same variable.

Example: A process has a datafield called X.  Since it is a process variable, there can only be 1 instance of that variable regardless of the process state. So in the rules, there are no logic in having this option enabled when checking if X = true.

If I have an activity variable however, technically every destination user has its own variable instance to manipulate. Used in cases like 2 managers need to approve the request. This can not be done using process fields, as the users will overwrite previous decisions (one instance remember). So to test the outcome, the logical function will check if "At least 2" instances of the activity variable was set to "Approved".