How can batteries be in series?
When electrons arrive at the cathode terminal of a battery, it doesn't matter whether they came from another battery or from some other kind of circuit element. All electrons are indistinguishable.

They contribute to the chemical reaction in the battery just the same, wherever they come from.

Similarly, excess electrons are generated at the anode terminal, and it makes no difference whether the wire connected to that terminal leads to another battery or another kind of circuit element.