score:6
According to the SNCF web-site there are 15 stations in France which offer left luggage facilities and Biarritz is not one of them.
https://www.gares-sncf.com/fr/gare/frbiq/biarritz/actualite/13301/consignes-bagages-gare
The page is in French but the list of stations is clear enough. Note that two of them offer a manual office the others are automatic lockers.
If you search for Biarritz consigne you will find there are commercial enterprises offering left luggage in Biarritz but since I have never used one I will not provide a link to a specific service. Be careful you do not find a restaurant called La Consigne by mistake.