This is not an error, this is because seating availability at a specific price is limited.
Tickets will have the same price for all the passengers on the same booking. If the number of seats available at a certain price is less than the number of passengers on your booking, it will automatically display the price of the next fare with sufficient available seats.
Take this into account at the time of reserving your flights. The lowest priced seats get taken the closer you get to the date of your flight, and as the plane fills up. If you want the best price guaranteed, don't wait and make your reservation as soon as possible.
Additionally, remember that on Volotea you can make group bookings from 10 and up to 100 passengers.