If there's no seats showing available on a Club Europe leg and you're not gold/OWE, it's likely the only seats left are in row 1 which is blocked to us mere normals. One of two things will happen, namely either the curtain moves back at some point creating more space, or it doesn't move and row 1 becomes available at OLCI.

(I wouldn't bother paying for pre-assignment of seats, especially on such a short flight, but it's your money...)