Most problems come from one of three places: the Google account you signed in with, a permission that was unticked at sign-in, or a source such as Bing or Yandex that has no sites in it yet. The pages below cover each case, with the exact message you will see.
If you are not sure where to start, begin with the connection checklist. It sorts the common messages by cause and sends you to the right page. You can also ask your assistant which sources are connected and what your subscription state is, and the answer often points straight at the problem.
Billing questions, such as how the trial ends and how to cancel, have their own page.