After skipping your first if block when server_queue doesn't exist, you are then trying to access playing on top of the nonexistent server_queue. Try adding and else if block instead of if to ensure you have it.

const server_queue = queue.get(interaction.guild.id);
        if (!server_queue) {
            const server_data = {
                queue: [],
                connection: null,
                dispatcher: null,
                playing: false,
            }
            queue.set(interaction.guild.id, server_data);
            server_data.queue.push(url);
        }
        else if (server_queue.playing) {
            return interaction.reply("")
        }