You don't need an account on every platform. To post in a campaign, you need a connected account on one of the platforms it accepts. A post from another platform is refused with Platform not in this campaign.
When you connect an account in Settings, you choose how:
To link Instagram, your account must be a Professional account.
By default, Instagram campaigns accept Reels and feed videos. Some campaigns also accept carousels and single images; the brief then says Reels & feed posts. Accounts added with a bio code can't submit carousels or images.
To submit, see How can I upload a video to a campaign?