Can’t download my Instagram after canceling the subscription on App Store

App re-download is not tied to app subscription.

What happens when you try to re-download the app from your previous purchases? Any errors?

Is your iPhone storage near capacity? If so, it is possible to have an app stuck between installed and uninstalled (a “stuck” app will usually show with a grid-like icon of fine traces, not the usual app icon), and the only fix for that is freeing some space, backing up, a factory reset, and restoring the backup.