Go to your Business Manager and click Business Settings > Instagram Accounts.
Click on the Instagram account you'd like to assign an ad account to.
Click Assign Ad Accounts.
To authorize one or more of your ad accounts to use the Instagram Account, check the box next to each ad account and click save changes.
It is also important that your brand's fanpage is linked to your Instagram account via the app.
For more info, you can visit Facebook business help.