Inconsistent Accessibility: Parameter type is less accessible than method

Constructor of public class clients is public but it has a parameter of type ACTInterface that is private (it is nested in a class?). You can't do that. You need to make ACTInterface at least as accessible as clients.

Make the class public.

class NewClass
{

}

is the same as:

internal class NewClass
{

}

so the class has to be public

If sounds like the type ACTInterface is not public, but is using the default accessibility of either internal (if it is top-level) or private (if it is nested in another type).

Giving the type the public modifier would fix it.

Another approach is to make both the type and the method internal, if that is your intent.

The issue is not the accessibility of the field (oActInterface), but rather of the type ACTInterface itself.