Why can’t I see the necessary Instagram account in the list when creating a Campaign?

It is likely that you assigned your Instagram account to your Business Manager (http://business.facebook.com), but didn’t assign it to your Ad Account as it is written here.

Please make sure that you have an Ad Account in Business Manager with assigned Instagram account.

• Create or check out the list of your Ad Accounts:

https://business.facebook.com/settings/ad-accounts

• Set up an Instagram account and assign it to your Ad Account:

https://business.facebook.com/settings/instagram/