In recent days Instagram made some modifications, now it is required to follow some steps to link your Business Instagram account to the platform.
1- Access business.facebook.com
2- Complete the requested data and register
3- Acces "Add page" section
4- Add a Page you own
5- Search for the page you want to link (or paste the link), it has to be the same that is linked to your Instagram Business profile
6- After you complete the process, you should be able to see a new "Business Manager" section