Due to PBE starter runes kit, you can have x18, x27, and so on in your rune inventory
1. Have x9 of (any): Armor Marks, Armor Pen. Marks, Damage Marks, Attack Speed Marks, Magic Pen. Marks, Armor Seals, AP Glyphs, CDR glyphs, MR glyphs, Mana regen glyphs, MR p/lvl glyphs.
2. Click on the store icon
3. In Featured Item, buy "PBE Starter Kit (Runes)" (Doesn't matter if you aren't "starter")
4. Check your runes inventory, there should be bugged runes.