$1L$ of $0.20M$ $NaOH$ and 0.10 mol $NaCH_3COO$ wouldn't form a buffer, because there is no acid to neutralize added bases.
Work Step by Step
$- NaOH:$ Strong base. $- NaCH_3COO$: Weak salt base. A buffer needs: 1. An acid and a base: Base: $NaOH$ and $NaCH_3COO$ Acid: Missing. Therefore, this mixture can't act as a buffer, because it doesn't have an acid, so it can't neutralize added bases.