Ruined portal structures generate with crying obsidian in the portal frame. However, crying obsidian itself cannot be actually used to make portals, which is considered intentional - see MC-173218. So it seems odd from a gameplay perspective that this block exists as part of the frame (especially considered from the perspective of a new player, who probably wouldn't understand why the portal they completed does not open).

Solution would be to either make crying obsidian not be part of the portal frame (potentially existing elsewhere in the structure instead), or allow it to be considered a valid frame material.