Looking for quickest and cheapest way to travel by rail preferably a high speed train, from San Sebastián to Barcelona,Spain
Thank you in advance
Your best option is to fly.
Some options shown here --- https://www.rome2rio.com/map/Donostia-San-Sebasti%C3%A1n/Barcelona
You can find rail schedules and fares on the Renfe website. Buying early usually saves money. Your departure station will be listed as San Sebastian/Donostia, and you'll arrive at Barcelona-Sants.
Thank you everyone for all your suggestions