Parisian local here
Since the start of the COVID pandemic you can’t buy tickets from the driver anymore (all have their separation shield out and ticket sales are not possible even though that’s still a theoretical possibility as mentioned by Willeke).
Nor can you use contact-less card (this will be supported eventually but only on the RoissyBus and OrlyBus specific airport lines, the authority is not keen on implementing that and prefers a confusing system)
If you have a French phone plan, you may use the facility of buying tickets by SMS
If you have a NFC-capable Android phone (this will not work on an iPhone), since a few weeks, you can buy and store tickets with the official IDF Mobilités app, note that you an also load day, week and months passes to it.
You validate then by tapping your phone on the Navigo purple or blue pads, here is the ones for buses and trams :
Source : By Greenski – Own work, CC BY-SA 3.0, Wikimedia Commons
In case you’re running empty on your card, you can also reload a physical Navigo Easy (or Navigo Découverte) with tickets (for the Easy only) and passes, using the aforementioned app
If you have an iPhone and can’t avail from the SMS facility, you can’t get on that bus.
Recent experience, travelling from Paris Orly airport to Four Peary. The bus didn’t had any option to pay cash. You have to take a ticket from a ticketing machine, or use a way to purchase online, or to use the mobile phone SMS (I think it will work only for French postpaid connections).
So, unfortunately, you may need to purchase a ticket before entering.
Credit:stackoverflow.com‘