This is more common than you might think. An agency, a collaborator, or even you at some point may have linked the page to a Business Manager. This can prevent you from connecting the page in Metricool even if you are an admin.
How to check:
Go to
business.facebook.comUnder Business Settings > Accounts > Pages, check if your page appears listed
If you don't have access to any BM, ask the other page admins to check from their accounts
If the page is inside a BM you don't have access to, you'll need to ask the BM owner to either add you as a user or unlink the page.