Note: If you are an Admin of your Facebook Business Manager, unfortunately, this does not mean you are automatically an Admin of the Facebook Page connected to your Instagram Business Profile.
Here is how to check if you are an Admin (or Editor) of the Facebook Page/Instagram profile you are trying to link to Iconosquare:
Step 1.) Please open Facebook in your internet browser, and sign in to your Facebook personal profile (this should be the profile connected to your Iconosquare account, ex. www.facebook.com/john.smith). Then click this link: https://www.facebook.com/bookmarks/pages?ref_type=logout_gear
Step 2.) Please select the Page you believe you have Admin/Editor access to. Next, click Settings on the top right-hand side, and then Page Roles on the left side. Scroll down; if your personal profile is not listed under ADMIN or EDITOR, unfortunately, you are not an Admin/Editor of that Page. You'll need to contact one of the other Admins to grant you access.