Did you attempt to switch your Instagram account to an Instagram Business Manager account after you were recently added to the business Facebook page as an administrator? If you attempted to do this within 1 week of being added to the Facebook page as the admin, you receive the following message.
As a security precaution, you aren’t allowed to approve a request to add this Page to a Business Manager account because you were added as an admin on the Page less than a week ago (until date).
This is an added security security measure Instagram uses to prevent someone from hijacking your Instagram account, since you were recently added as an administrator to the linked Facebook page. The grace period allows Instagram to review potential malicious activity associated with the Facebook or Instagram account and take action. Simply wait the allotted week and go back and convert your Instagram account to a business account.
How to: switch to an Instagram Business account.