can't find flights

Where does the tour start and end? If you can't find flights to those place, fly somewhere close and take the train the last bit.

Most people here don't know the specifics of tours.
Tour starts in Bayonne and ends in Bilbao.

We flew to Paris, then took the train to Bayonne. After the tour we took the train to Madrid for a few days before flying home.

What is your home airport?

Fly to Madrid and pick up a flight to Bilbao, San Sebastián……

Bayonne doesn't have an airport, so good luck finding flights there. Closest is nearby Biarritz, but that is hardly a major airport. There are connections to a couple of large hubs, but not that many. And it might be easier to fly to Paris and take the train to Bayonne.

Bilbao has an airport, but not a big one so you might consider flying home from Madrid.

It’s a 4 to 5 hour train ride between Madrid and Bilbao, if Madrid becomes part of the equation.

The most important airport is Bilbao, from there you have an easy access practically anywhere in the Basque Country. For Bayonne, Id try Biarritz or Donostia -San Sebastian airport, but they are very small with few flights.

If you aren't aware of it, for first layer research, rome2rio website can give you your logistical options from point A to B. Don't rely on it for price and times, but it should give you options of where you can fly to and then train or bus.

Here is what the Rick Steves tour website says about flights for this tour:

https://www.ricksteves.com/tours/spain/basque

Our tours don't include airfare, so your flight arrangements and transfers are completely up to you.

Most tour members arrange to fly into Biarritz, France (BIQ), and out of Bilbao, Spain (BIO) — choose "multicity" on flight-booking websites. The Biarritz airport is a 20-minute bus ride from Bayonne, where the tour begins. Booking round-trip to one city will require a connecting flight, train trip, or additional overnight stay in Europe.

We suggest arriving a day or two before your tour begins to allow ample time to get over jet lag and be well-rested for the first few days of your tour. For more information about transportation options for getting to and from your tour, see your Rick Steves Snapshot: Basque Country guide.

Rome to Rio indeed is first layer research to a load of absolute nonsense.

So this is my 'first layer research'-

If the RS website says "most tour members arrange to fly into Biarritz" then it makes sense to look at the airport website-
Here is their flight schedule in French (so 4 connections a day from Paris)- https://biarritz.aeroport.fr/wp-content/uploads/2023/02/programme-ete-2023.pdf

Here is the bus schematic from the airport website-
https://biarritz.aeroport.fr/en/parking-transport/buses/
That website links to the schedule for the buses 3 and 4-
https://itineraires.txiktxak.fr/fr/horaires/TXIK-TXAK-Nord/Bus/ligne/4/direction/OUTWARD/4
(which cost 1.2 Euro)

Here is the Air France Schedule for a sample date in mid May-
https://wwws.airfrance.co.uk/search/offers?gclid=60260a51d2621758eeb56e527f98348e&gclsrc=3p.ds&pax=1:0:0:0:0:0:0:0&cabinClass=ECONOMY&activeConnection=0&connections=CDG:A:20230517%3EBIQ:A&bookingFlow=LEISURE

And here is the SNCF train schedule for Paris to Bayonne for a sample day in mid May
https://www.sncf-connect.com/app/home/shop/results/outward

If you look at Bilbao Airport website you will see that, amongst other places, there are flights to Paris CDG on Vueling Airlines and Air France-
https://www.aena.es/en/bilbao/airlines-and-destinations/airport-destinations.html

and that you get there from the City Centre on a bus which runs every 20 minutes, and costs 3 Euros
http://apli.bizkaia.net/APPS/DANOK/TQ/DATOS_PARADAS/ITINERARIOS/A3247I.PDF

Another option is to fly round trip to either Madrid or Bilbao.
Alsa run regular coaches from both cities to Biarritz and Bayonne, and also between Bilbao and Madrid City Centre and direct from Bilbao to Madrid Airport-

https://www.alsa.com/en/web/bus/home

If flights to Biarritz (BIQ) do not work out well for you, you can also look at flights to Bordeaux (BOD): it is not as convenient, but it could be cheaper as there are a bit more flights to Bordeaux (including some from other European hubs like Frankfurt & Amsterdam).