Instagram only allows scheduling posts from Creator or Business Instagram accounts. So to connect your account to Postpone, you'll first need to convert it to either a Creator or Business account.
Creator Instagram Accounts
Creator accounts are best for individual content creators.
To convert your Instagram account to a Creator account, follow the steps in this Instagram help article:
How to set up a Creator account on Instagram
Business Instagram Accounts
Business accounts are best for businesses and products.
To convert your Instagram account to a Business account, follow the steps in this Instagram help article: