Instagram no longer requires a connected Facebook Page to publish posts via apps like Postpone. However, if you would like more in-depth analytics in Postpone, then you'll need to link your Instagram account to a Facebook Page.
Prerequisites
Before linking a Facebook Page to your Instagram account, you must first convert your Instagram account to a Creator or Business account.
Creating a New Facebook Page
If you don't have a Facebook Page yet, you can create one directly from Instagram. Follow the steps in the official Instagram guide:
How to create a Facebook Page from Instagram
Connecting an Existing Facebook Page
If you already have a Facebook Page, you can link it to your Instagram account. Follow the steps in the official Facebook guide: