We couldn't go for a walk because of the storm.

Literal

Storm [because-of-ため] walk [to-に] go-could-not.

散歩に行く is a fixed collocation ('go for a walk') where に marks the purpose/destination of motion — 'go [for the purpose of] a walk.' The potential negative 行けなかった is the standard 行く→行ける→行けない→行けなかった derivation for u-verbs.