Option 1 - Ask around!:
To complete the sign up process, someone with full control of the Facebook business account that is associated with your page must grant the required permissions. Ask your colleagues if they set up or know who may have set up the business account & page.
Option 2 - Set up a new page:
If you wish to participate in campaigns while you attempt option 1, you can simply create a new page with the purpose of taking part in the campaigns by following the steps outlined here: https://www.facebook.com/business/tools/meta-business-suite/get-started
Option 3 - Contact Meta to reclaim your page:
If you do not know who has full control over the page or business account, or the original admin is no longer with the company - you can submit a request to Facebook to reclaim the page along with the full admin rights by following the instructions here: