kwant.builder.Lead is an abstract base class. This means it cannot be instantiated directly, it’s purpose it to be subclassed. Instead, you may create instances of its “children”: kwant.builder.BuilderLead, kwant.builder.SelfEnergyLead, or kwant.builder.ModesLead. You probably want to use the first of these.
It should be all clear when you read https://kwant-project.org/doc/1/reference/generated/kwant.builder.Lead and https://kwant-project.org/doc/1/reference/generated/kwant.builder.BuilderLea...
Do let us know if any questions remain.