Hi, FCFF is not impacted by leverage because this is the CF available to both suppliers of debt and equity, so you don’t want to account for debt here yet. Look at the formulas, it purposely excludes interests. If you included leverage in FCFF, then you wouldn’t be able to see how much is left/available to debt-holders.
In contract, FCFE is the CF available to suppliers of capital only, so there you want to account for debt since shareholders get paid after debt-holders.