To use Instagram DMs and comments, you need an Instagram Business Account. If it's not enabled on your brand's IG profile, here's how you can do it:
1) In the Instagram mobile app, go to your profile.
2) Click ☰ in the top right corner.
3) Click Settings > Account.
4) Click "Switch to Professional Account" or "Switch to Business Account" at the bottom.
5) When given the option between Creator and Business, tap "Business" and [Next].
6) Connect your Instagram business profile to an existing Facebook Page (your brand page) or create a new Facebook Page.
7) Review your business contact information and click on [Done].
Alternatively, if you prefer to read the same instructions directly from Instagram, here's their official help center article.