Since June 15th, 2021, as part of Facebook's new features, it's required that the Off-Facebook Activity setting is on to connect to third-party apps, like Campsite.bio.
Here are some steps from Facebook that may resolve the issue:
Ensure that you've toggled your Off-Facebook Activity on. More info on what this is and where to find it here - example screenshot below:
Confirm that all admins on the associated Business Manager account have the "off-facebook activity" feature toggled on. Not sure which Business Manager account owns the Facebook Page? Follow the steps in this Facebook Help Guide.
Confirm that your Facebook page is connected to your Instagram profile in both Facebook and Instagram. Not sure how? Follow the steps in both of these guides: Instagram How To Guide Linked & Facebook How To Guide Linked
Once all of the above has been confirmed, please try to add or reconnect your Instagram account again
If you have any questions, please contact our customer support team by emailing firstname.lastname@example.org.