To connect an Instagram account to funnel a few criteria has to be met.
The Instagram account has to be a business account. It also has to be connected to a Facebook page. Once the Instagram business account is connected to a page, any Facebook user who is able to perform tasks on behalf of that page can connect the Instagram business account to Funnel.
An Instagram account can be converted to a business account at any time.
If you have connected with the Media insights report, any media that was posted before the Instagram account was converted to a business account will have zero values for the following metrics:
Impressions
Reach
Engagement
Saved
Carousel Album Impressions
Carousel Album Reach
Carousel Album Engagement
Carousel Album Saved
If you have connected with the Account insights report there will only be data available from the time when the account was converted to a business account.
You can read more about common issues that may arise when connecting to Instagram Insights in Instagram Insights - Troubleshooting