Learn Russian: Твой календарь пустой, почему ты не едешь в офис? - Your calendar is empty, why aren't you going to the office?

Nouns ending in a soft sign can be either masculine or feminine. Here, you can tell календарь is masculine because the words describing it—твой (your) and пустой (empty)—are in their masculine forms.

Why does this sentence use едешь for "going" instead of идёшь?

In Russian, verbs of motion are very specific. Идёшь means going on foot, while едешь means going by transport (like a car, bus, or train). The speaker assumes you would need to commute to the office using some kind of transport.

Why is it в офис and not в офисе?

We use в + the Accusative case to show direction or destination (going to somewhere). Because офис is an inanimate masculine noun, its Accusative form looks exactly like its dictionary form. If you were already at the office, you would use the Prepositional case: в офисе.

Since the current lesson is about double negatives, why is there only one не here?

Double negation is required only when you use negative pronouns or adverbs like ничего (nothing) or никогда (never). Because this sentence just asks a simple "why aren't you going?", there are no "nothing/never" words involved. You just put a single не directly in front of the verb: не едешь.