I tried to craft a blue firework star, eveything in line with the information on Minecraft Wiki, but when the ingredients were fitted in the crafting table, the slot on the right side remained empty.

I had some fireworks crafted this way in an older version, and these rockets are normally working, except that they disappear when they are obstructed by something above it. Rockets that I crafted in 1.15 are behaving normally when they are blocked by an object above.