To unlock this mod, you first need to purchase the EPIC mods (open the branches) in the top and bottom rows.

Pay attention to the guide lines — they show exactly what needs to be unlocked. To unlock the EPIC mid-row mod, you must also unlock and purchase the top and bottom ones, while the top and bottom mods don’t depend on others.

The EPIC mod in the middle of the tree unlocks access to LEGENDARY mods, which are the ultimate upgrades. That’s why it takes more effort to obtain.